WASHINGTON — Iran has established a drone airfield inside Syria not far from a U.S. military base, U.S. officials said Tuesday. The airfield may be the source of drones that have recently threatened U.S. troops.

According to two U.S. defense officials, the Iranian airfield near Palmyra, Syria, has a ground control station for operating drones. The U.S. and the coalition have a base in At Tanf, Syria, close to where the Iraqi, Jordanian and Syrian borders intersect, 80 miles south of Palmyra.

Iranian drone Shahed-129 is displayed by armed forces in a rally commemorating the 37th anniversary of the Islamic revolution February 11, 2016 in Tehran. Ebrahim Noroozi / AP File

The Iranians have been basing Shahed-129 drones at the airfield, according to the officials. The Iranian-made drones can be armed or used for reconnaissance. Neither official could say for sure whether either of the two drones that the U.S. has shot down in southern Syria flew out of that base, but both said it was likely.

Related: U.S. Shoots Down Syrian Jet

The U.S. has established a 55-kilometer de-confliction zone around the base, and warned that any craft entering it may be intercepted. They recently deployed rocket launchers, called HIMARS, at the base to support Syrian Democratic Forces operating in the area and to provide protection to the garrison.

On June 8, the U.S. shot down a drone that had fired on coalition forces near At Tanf. A second armed drone was shot down by a U.S. jet on Tuesday as it approached coalition forces in the same area. The U.S. also downed a Syrian government warplane earlier this week in northern Syria.

Neither official would speculate on who is operating the drones that have tried to attack U.S. and coalition assets, but both said the Iranians have been training the Syrians how to use them.

U.S. special forces have been training Syrian opposition fighters at the At Tanf base for more than a year.