American F-15s and Apache helicopters carried out a "show of force" in Syria on Tuesday after Turkish-backed militia fighters "came very close" to a U.S. base, according to a U.S. official.
The incident underscores the complex battlefield in Syria as 1,000 American troops are withdrawing from the country in the middle of fighting between Turkish forces and Syrian Kurds.
The U.S. official said the incident occurred near Ayn Issa, a town 18 miles from the Turkish border, where U.S. troops withdrew from their positions earlier this week.
According to the official, Turkish-backed militia fighters "came very close" to a base used by U.S. and Kurdish forces, putting the U.S. forces at risk.
The Turkish-backed fighters had violated an agreement with the U.S. to not get too close to U.S. forces and threaten them, the official added.
Typically, aerial shows of force involve aircraft flying at low altitude above opposing forces to demonstrate potential strength, should it be required.
The U.S. military also formally contacted the Turkish military to protest the risk posed to the American forces by the nearby presence of Turkish-backed fighters.
The 1,000 American forces have begun to withdraw from the battlefield areas in northern Syria. Earlier on Tuesday, Russian military forces appeared in Manbij after the withdrawal of the estimated 100 American troops stationed in that town.
A second U.S. official told ABC News the withdrawal of American troops and equipment in northeast Syria could take weeks. Most troops are expected to be withdrawn via aircraft. Equipment will be shipped out via air and by land.
A third U.S. official told ABC News additional military forces will have to enter Syria in order to transport out the equipment. This includes transport vehicles and whatever forces are required for protection.
According to the official, the U.S. has made contact with the Russian military, Kurdish forces and the Turkish military to ensure they're aware that the U.S. military troops in Syria are focused on a withdrawal.
Under the withdrawal plan authorized by President Donald Trump, a small contingent of U.S. military forces will remain at At Tanf Garrison, a military facility near the border with Jordan and Iraq.
In a statement issued on MondayTrump said the small force would remain to deal with remaining ISIS fighters, but U.S. officials have previously acknowledged the main purpose of the force at At Tanf is to check Iranian military weapons flows and advances into Syria.