WASHINGTON (Reuters) - The United States launched an air strike on Tuesday against Iranian-backed fighters who it said posed a threat to U.S. and U.S.-backed forces in southern Syria, in a new escalation of tensions between Washington and troops supporting Damascus.

The Pentagon, which has sought to stay out of Syria’s civil war to instead focus its firepower on Islamic State militants in Syria and Iraq, carried out a similar strike on May 18 that was denounced by Damascus.

A Syrian military source said the strike had caused deaths and material damage and showed the coalition was “in support of terrorism”. The Syrian military command warned against the dangers of escalation, the source added.

In recent days, the U.S. military has repeatedly warned massing forces to stay away from a so-called “de-confliction” zone near a garrison used by American special forces and U.S.-backed fighters around the southern town of At Tanf.

The zone was agreed with Russia, Syrian President Bashar al Assad’s ally. Assad is also backed by Iran and Shi’ite militias.

“Despite previous warnings, pro-regime forces entered the agreed-upon de-confliction zone with a tank, artillery, anti-aircraft weapons, armed technical vehicles and more than 60 soldiers,” the U.S.-led coalition said in a statement.

It said the United States, via a military hotline with Russia, issued several warnings before the strike, which destroyed two artillery pieces, an anti-aircraft weapon and damaged the tank.

A U.S. official, speaking on condition of anonymity, confirmed the strike was carried out by manned, U.S. aircraft.

“The Coalition does not seek to fight Syrian regime or pro-regime forces but remains ready to defend themselves if pro-regime forces refuse to vacate the de-confliction zone,” the statement said.

Still, the incident showed that the area around the Tanf garrison in southern Syria was under growing pressure.

BAGHDAD-DAMASCUS HIGHWAY

Tanf is part of a region known as the Badia, which consists of vast, sparsely populated desert territory that stretches all the way to the Jordanian and Iraqi borders and was declared a military priority by Syria’s foreign minister earlier in May.

U.S.-backed rebel advances against Islamic State militants have allowed them to secure swaths of territory in the Badia, alarming the Syrian government and its allies.

U.S.-backed rebels took Tanf from Islamic State last year, and regional intelligence sources say they mean to use it as a launchpad to capture Bukamal, a town on Syria’s border with Iraq and an important jihadist supply route.

The coalition’s presence in Tanf, on the Damascus-Baghdad highway, was also meant to stop Iran-backed groups from opening an overland route between Iraq and Syria, the sources say.

In interviews with Syrian state-owned TV station al-Ikhbariya broadcast on Saturday, Syrian soldiers vowed they would reopen the Baghdad-Damascus highway.

“Within a few days, you will find us at the Iraqi border, and we will achieve a great and historic victory in the history of this crisis - cleansing the entire Badia, liberating the Tanf road ... and restoring the vital artery between Iraq and Syria,” said one of the soldiers interviewed by the state TV channel in the Badia area.

Western-backed rebels resumed on Tuesday a heavy barrage of rocket attacks on Iranian-backed militia outposts along the Baghdad-Damscus Baghdad highway.

The United States sought to keep all sides focused on battling Islamic State, or ISIS.

“The Coalition calls on all parties in southern Syria to focus their efforts on the defeat of ISIS, which is our common enemy and the greatest threat to regional and worldwide peace and security,” the statement said.