Vice President Mike Pence and Secretary of State Mike Pompeo will meet with President Recep Tayyip Erdogan of Turkey in Ankara on Thursday to relay President Trump’s demand that Mr. Erdogan negotiate a cease-fire in Syria and to reiterate the president’s threat to impose economic sanctions if he does not comply.
The president’s sending of the delegation — to also include his national security adviser, Robert C. O’Brien, and the State Department’s special envoy for Syrian affairs, James F. Jeffrey — is part of a scrambled effort to wrest back control of a chaotic situation in Syria that has endangered American forces there even as they prepared to leave.
“We want to bring our soldiers back home after so many years,” Mr. Trump said in an appearance on Tuesday in the Rose Garden. “And they’re the greatest warriors in the world.”
Mr. Trump surprised his own military when he ordered the immediate withdrawal of all 1,000 American troops in northern Syria, who had been fighting Islamic State militants alongside Syrian Kurds. The forces’ withdrawal, ahead of Turkey’s cross-border offensive, has prompted the Syrian Kurds to turn for support to the Syrian government of President Bashar al-Assad and the Russians.
The risk to American troops became evident on Tuesday, when the American military ordered Apache helicopter gunships to conduct a low-flying show of force to warn off Turkish-backed militia members who were approaching, an American military official said.
Mr. Trump has long viewed his personal relationships with leaders, including Mr. Erdogan, as key to accomplishing his foreign policy goals. But Mr. Trump’s failure in an Oct. 6 phone call to outline the consequences for Mr. Erdogan if he moved his troops into Syria has forced the White House to deal with the consequences.
“The administration is resolved to maintain security in the region, the safety of civilians, and the continued detention of ISIS fighters,” Mr. Pence said Tuesday in a statement.
Mr. Pence does not have broad experience negotiating with Mr. Erdogan. This trip will be his first diplomatic visit to Turkey, but he has been repeatedly used as an emissary for Mr. Trump.
The addition of Mr. Pompeo, who has maintained that the president did not give Mr. Erdogan a “green light” to proceed into Syria, is intended to add diplomatic weight to the proceedings. And Mr. O’Brien, in his former role as the State Department’s hostage negotiator, helped secure the release last fall of Andrew Brunson, an American pastor who had been detained in Turkey for two years, leading to a diplomatic truce between the two countries.
Administration officials have sought in recent days to play down the idea that Mr. Trump’s decision to pull American troops from northeast Syria had contributed to a rapid destabilization of the area. One senior official likened it on Monday to a matter of moving “a couple of dozen guys around.”
But developments on the ground reflected growing security concerns for United States military officials stationed there.
In describing the Tuesday episode in Turkey, officials said that Turkish-backed fighters, who have formed the vanguard of the offensive, violated an agreement not to threaten American troops, who were about four miles away, when they advanced west of Ain Issa, Syria. The official spoke on condition of anonymity to discuss confidential intelligence reports.
The episode, which was first reported by Fox News, represented the second time in a week that Turkish forces or their proxies had threatened American forces caught in the mayhem of the offensive. American troops came under artillery fire last Friday from Turkish positions. The artillery struck a few hundred yards from the Americans, whose location had been relayed in advance to the Turkish military command.
Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, said on Friday that commanders up and down both American and Turkish chains of command had been alerted to where American forces were. “Everyone has been told,” General Milley told reporters at the Pentagon.
At the same time, American troops across northeastern Syria continued to flow south, packing up equipment and weaponry, and moving away from the advancing Turkish offensive. A video posted online Tuesday showed a Russian journalist standing in an abandoned American outpost west of Manbij that was the closest one to Syrian government troops.
A spokesman for the American-led coalition based in Baghdad confirmed that withdrawal on Tuesday on Twitter. “We are out of Manbij,” wrote the spokesman, Col. Myles B. Caggins III.
The United States has pulled out of about half a dozen bases or outposts in the past week, and continues to operate at eight to 10 more outside the immediate conflict zone. Military officials described one incongruous scene in which Syrian government forces — which this week struck a deal with Syrian Kurds, the erstwhile American ally — advanced north to fill positions vacated by American troops moving south on the same highway.
The Pentagon has not received a final deadline for pulling troops from northern Syria. For the moment, the military is planning on “weeks, not days” to complete the withdrawal, according to one officer briefed on operational planning. Most of the American troops are expected to leave by transport planes or helicopters, or by convoy into neighboring Iraq.
A small number of allied Special Operations forces from countries like Britain and France that have been conducting missions in the northeast alongside the Americans are expected to leave under the United States logistical umbrella, officials said.
Hundreds of American troops in the region are standing by to provide security for the retreating ones now that most Syrian Kurdish forces, which had provided security at bases and roadways, have been redirected north to confront the Turkish invasion, military officials said.
Dozens of American warplanes and armed drones that had been conducting other missions, including counterterrorism operations, have in the past several days been reassigned to watch over the departing troops.
Defense Secretary Mark T. Esper said in a statement on Monday night that the Turkish incursion had “resulted in the release of many dangerous ISIS detainees.”
But an American military official said that the Syrian Kurds, using minimal manning levels, continued to detain more than 10,000 ISIS fighters, including 2,000 foreigners, at roughly two dozen makeshift prisons around the area.