WASHINGTON — Military leaders were scrambling Thursday to devise a swift but safe departure of troops from Syria as outraged lawmakers appealed to President Donald Trump to reverse an abrupt withdrawal order that rattled Washington’s Kurdish allies and raised questions about America’s perceived reliability as a wartime partner.

Defense officials said U.S. airstrikes would continue until all the approximately 2,000 U.S. troops are out of Syria, but it was unclear whether the air campaign would then end. Officials said it might depend on whether France and other coalition partners keep ground troops in Syria after the Americans leave. A continued presence of allied troops working with local Syrians might compel the U.S. to contribute air cover.

The Pentagon offered no information about how the withdrawal will happen or how long it will take, apparently because they don’t know.

Two officials speaking on condition of anonymity to discuss internal planning said Gen. Joseph Votel, head of U.S. Central Command, will submit a comprehensive withdrawal plan to top Pentagon officials in coming days.

One official said military commanders are concerned that the pullout will leave their Syrian Kurdish allies in the lurch.

Turkey, which considers the Kurds a security threat, has said it intends to send its army into Syria to clear them out. Until now, Turkey had to consider the presence of U.S. troops; once they are gone, a bloody clash seems inevitable.

Sen. Lindsey Graham, R-S.C., said Thursday that Defense Secretary Jim Mattis had told him he is worried about the Kurds and this is not the right time to leave.

“The day we leave it’s going to be open season on every Kurd who’s supported us,” Graham told a Capitol Hill news conference, adding that the Syrian regime, the Turks and Islamic State fighters will go after the Kurds “like a pack of wolves.”

The Kurdish-led Syrian Democratic Forces, who have partnered with U.S. troops for years as the key force against IS militants, said in a strongly worded statement laced with bitterness that the fight against IS was at a “decisive” stage that requires even more support from the U.S.-led coalition.