WASHINGTON —Iran is believed by the US to be responsible for last week's attack on Al-Tanf, a base in southern Syria where US troops are located.

US officials confirmed that five drones were launched from within Syria in the attack on Al-Tanf.

They said on Monday the US believes that Iran resourced and encouraged the attack, but that the drones were not launched from Iran. They were Iranian drones, and Iran appears to have facilitated their use, officials said, speaking on condition of anonymity to discuss details that have not been made public.

Pentagon spokesman John Kirby declined to provide details when asked about the report during a news conference Monday. He called it a “complex, coordinated and deliberate attack” and said the US has seen similar ones before from Shia militia groups that are backed by Iran. But he would not go into specifics and said he had no update on the munitions used in the attack.

No American troops were injured or killed in the attack on the base, located near the borders of Iraq and Jordan, but bomb fragments were later discovered in areas where troops sleep and stand card.

There are about 200 US soldiers at Al-Tanf, which is controlled by Syrian opposition forces. US and coalition troops are based at al-Tanf to train Syrian forces on patrols to counter IS militants. The base is also located on a road serving as a vital link for Iranian-backed forces from Tehran all the way to southern Lebanon and Israel.

Kirby also declined to say if troops were warned ahead of time or whether the U.S. intends to make a military response.

“The protection and security of our troops overseas remains a paramount concern for the secretary,” Kirby said, referring to Defense Secretary Lloyd Austin, “and that if there is to be a response, it will be at a time and a place and a manner of our choosing, and we certainly won't get ahead of those kinds of decisions.”

Pro-Iran media outlets have been saying that the attack on Tanf was carried out by “Syria’s allies” — an apparent reference to Iran-backed groups — in retaliation for an attack days earlier near the historic Syrian town of Palmyra. Israel has been blamed for the attack, but US officials say America was not involved with it.

The Al-Tanf attack came in a period of rising tensions with Iran. The Biden administration this week said international diplomatic efforts to get Iran back into negotiations to return to a 2015 nuclear deal were at a “critical place” and that patience Is wearing thin.

About 900 US troops remain in Syria to assist efforts to combat the IS. — Agencies