WASHINGTON — The Trump administration called on Turkish President Recep Tayyip Erdogan to implement an immediate cease-fire in northern Syria and imposed sanctions against Turkey on Monday in response to its military aggression as the situation on the ground continued to deteriorate after President Trump’s decision to withdraw U.S. forces.

Vice President Mike Pence announced that he and national security adviser Robert O’Brien would lead a delegation to Turkey in the “immediate future” in an effort to end the violence in the region that has increasingly become a political problem for Trump at home.

Pence said that Erdogan and Trump spoke by phone on Monday and that the U.S. president “communicated to him very clearly that the United States of America wants Turkey to stop the invasion, to implement an immediate cease-fire and to begin to negotiate with Kurdish forces in Syria to bring an end to the violence.”

Trump has faced intense criticism, including from leading Republicans, for his decision to pull the troops, and he has been under pressure to get Turkey to back off its military incursion, which has targeted Kurdish fighters who have aided the U.S. fight against the Islamic State, also known as ISIS.

“For years, the United States and our Syrian Kurdish partners have fought heroically to corner ISIS and destroy its physical caliphate,” Senate Majority Leader Mitch McConnell, R-Ky., said Monday. “Abandoning this fight now and withdrawing U.S. forces from Syria would re-create the very conditions that we have worked hard to destroy and invite the resurgence of ISIS.”

The sanctions are aimed at Turkey’s ministries of defense and energy, as well as three senior Turkish officials. Among them was the interior minister, a powerful position responsible for domestic security.

Despite the sanctions move and tough rhetoric from administration officials, Trump continued to defend his decision to pull U.S. troops from Syria even though he was warned in advance that it would result in the mayhem occurring now.

“After defeating 100% of the ISIS Caliphate, I largely moved our troops out of Syria. Let Syria and Assad protect the Kurds and fight Turkey for their own land.” Trump tweeted, referring to Syrian President Bashar Assad. “I said to my Generals, why should we be fighting for Syria and Assad to protect the land of our enemy? Anyone who wants to assist Syria in protecting the Kurds is good with me, whether it is Russia, China, or Napoleon Bonaparte. I hope they all do great, we are 7,000 miles away!”

Vice President Mike Pence, with Treasury Secretary Steven Mnuchin, speaks to reporters outside the West Wing of the White House on Monday. The U.S. is calling for an immediate cease-fire in Turkey’s strikes against Kurds in Syria, and is sending Pence to lead a mediation effort. Jacquelyn Martin/Associated Press

Along with the sanctions, Trump said that tariffs on steel imports from Turkey will be raised 50 percent, and that the United States has halted negotiations over a $100 billion trade deal with the country.

The Treasury Department said the sanctions are not meant to disrupt humanitarian aid to Syrian civilians, but some aid groups already have started to pull their workers out of northeastern Syria.

As Syrian Kurds of the Syrian Democratic Force, or SDF, battled Turkish government troops and their allied militias at various points along the border Monday, Syrian government forces loyal to Assad began entering border cities at Kurdish invitation, under an agreement brokered by Russia. Fears rose across the region that an all-out war could start between Turkey and the Syrian troops, ultimately involving Russia and Iran, Assad’s primary backers.

Pence and Treasury Secretary Steven Mnuchin addressed reporters late in the afternoon and attempted to defend the president from criticism that he essentially gave Erdogan a green light to invade northern Syria during an Oct. 6 phone call.

The White House released a statement after the call that said Erdogan informed the president that Turkey “will soon be moving forward with its long-planned operation into Northern Syria” and that the United States did not support the move. The statement, however, made no mention of what Trump would do to oppose or stop Turkey’s aggression.

“President Trump made it very clear that the United States is going to continue to take actions against Turkey’s economy until they bring the violence to an end,” Pence said of the two leaders’ Monday phone call.

Trump was told on Monday afternoon by advisers that it would be costly not to do anything, that the absence of the United States from the region could strengthen Iran, and that the deteriorating situation could hurt him politically, according to people familiar with the discussions who spoke on the condition of anonymity to describe the private conversations.

People welcome Syrian troops as they enter the village of Ghebesh, west of the town of Tal Tamr, in northern Syria on Monday. Syrian government troops moved into towns and villages in northern Syria, setting up a potential clash with Turkish-led forces advancing in the area as long-standing alliances in the region begin to crumble following the pullback of U.S. forces. Associated Press

It “has been a days-long effort to get him in a better place,” one official said of Trump, adding that Pence and Secretary of State Mike Pompeo have been key to that effort.

The sanctions are “designed to focus Turkey’s attention on the gravity of the situation in northeast Syria,” said a senior administration official, one of several who briefed reporters on the day’s actions on the condition of anonymity imposed by the administration.

Pence and O’Brien, the official said, would “depart for Ankara as soon as possible to see if we can achieve a deal,” including an immediate cease-fire and the start of negotiations.

Asked whether Trump had provoked the Turkish incursion in the first place by signaling an initial withdrawal of U.S. troops, one official responded sharply that “this was something caused by the action of President Erdogan, after repeated warnings that this was a bad idea, and that the United States in no way endorsed this activity.”

The small number of U.S. troops in Syria, this official said, was not in a “position to stop an invading army.” Another official noted that “rather than them being encircled, and potentially being in the crossfire . . . we had to focus on [the troops’] protection.”

Trump’s concern now, this official and others said, was possible harm to civilians, and the escape of Islamic State detainees being guarded by Kurds who have turned their attention to Turkey.

An official said the United States has limited information on the current status of about 10,000 detainees, approximately 2,000 of them from other countries. “We don’t have a large footprint in Syria; we can’t be everywhere and know everything,” the official said. Indicating that most of the official’s information came from open media sources, the official said “we can’t provide confirmation of specifics on the number of potential detainees that may have escaped.”

Asked whether, in the Monday call, Erdogan had given Trump any indication that he was prepared to acquiesce to demands for a cease-fire and negotiations, a senior official said “the president would not be willing to send a high-level delegation on short notice like this unless he was pretty confident there was at least a chance of getting a cease-fire.”

Last week, when Trump offered to mediate between the Syrian Kurds and Turkey, Turkish officials firmly refused, saying the United States would not negotiate with “terrorists” and shouldn’t expect Turkey to do so.

The administration has faced bipartisan pressure from Capitol Hill to impose sanctions against Turkey, and Trump’s decision to pull out the troops has prompted an unusual outcry among GOP lawmakers who have otherwise hesitated to criticize the president.

In a tweet Monday, Rep. Adam Kinzinger, R-Ill., called Trump’s approach to Syria “weakness” and added: “America is far more honorable than this.”

The widespread anger on Capitol Hill has created unusual alliances. House Speaker Nancy Pelosi, D-Calif., spoke with Sen. Lindsey Graham, R-S.C., a close Trump ally, earlier Monday and said the two agreed to pursue a congressional resolution “to overturn the president’s dangerous decision in Syria immediately.”

The prime goal of the resolution, according to one aide familiar with the strategy, is to show a “strong, bipartisan consensus that the president’s decision must be overturned” and force Trump to either sign it or veto the measure.

Pelosi and Graham also agreed to work on a sanctions measure, with the speaker insisting on a package that was “stronger” than what had been proposed by the White House. Arguing that Trump has “unleashed an escalation of chaos and insecurity in Syria,” Pelosi said the plan unveiled by the administration late Monday “falls very short of reversing that humanitarian disaster.”

Graham, who was at the White House on Monday, told Trump that getting Democrats and Republicans to agree on sanctions would show they are popular, which is key to getting him to sign the legislation.

Democrats continue to hammer Trump over his decision and said he is now responsible for cleaning up a mess he created.

“As Congress works to counter the president’s reckless decision, the only person able to immediately stop this tragedy unfolding is the president himself,” three top Senate Democrats – Minority Leader Chuck Schumer of New York; Robert Menendez of New Jersey, the ranking Democrat on the Foreign Relations Committee; and Jack Reed of Rhode Island, the ranking Democrat on the Armed Services Committee – said in a joint statement Monday.

The pending trade agreement that Trump said would be halted Monday was one element of a package that the Untied States had offered Turkey before the incursion began. It was an attempt to prevent the offensive and repair relations between the two countries.

The Office of the U.S. Trade Representative has estimated that total trade between the United States and Turkey was $24 billion in 2017. That figure is nowhere near the $100 billion referenced by Trump, although in June, the president said at the Group of 20 summit in Japan that the United States was looking to quadruple its trade with Turkey as he met with Erdogan.

Commerce Secretary Wilbur Ross spent several days in Istanbul and Ankara last month in meetings with Turkish officials aimed at smoothing commercial relations between the NATO allies. He was accompanied by representatives of the U.S. Chamber of Commerce and about 15 companies, including members of the Fortune 100.

But some experts speculated that expanding trade with Turkey by that magnitude was already unrealistic, taking some teeth out of the threat to halt trade talks between the two countries.

“It is dishonest to then claim we won’t do what was impossible anyway and feel that we are punishing Erdogan,” said Michael Rubin, an analyst at the American Enterprise Institute. “That’s the trade equivalent of putting Turkey on double-secret probation.”

The Washington Post’s Josh Dawsey, Dan Lamothe, David Lynch, Carol Morello and Felicia Sonmez contributed to this report.