Pentagon declined to speculate on who was operating it but said drone advanced on coalition forces in Syria’s southeast.
The US military says it has shot down an armed, Iran-made drone that had been bearing down on its forces near a garrison in Syria’s southeast.
In the latest sign of increasingly frequent confrontation with Damascus and its allies, Tuesday’s incident closely followed Sunday’s downing of a piloted Syrian army jet by the US in the southern Raqqa countryside after it dropped bombs near US-backed forces.
The Pentagon said a US F-15 aircraft, flying over Syrian territory, fired on the drone after it displayed hostile intent and advanced on coalition forces.
Pentagon spokesman Captain Jeff Davis said it had "dirty wings", meaning it was armed.
"I can tell you it was an Iranian-made drone," Davis said, declining to speculate on who was operating it.
The area falls in a part of Syria that was recently identified as a military priority by Damascus, and strategically important for Iran as it seeks to secure a land corridor between forces it backs in Syria and Iraq.
The US-led coalition said the location was close to where another "pro-[Assad] regime" drone, which intelligence sources had also identified as Iranian, was shot down on June 8.
The drone was shot down after it dropped bombs, aimed only at dirt, but in an area close enough to coalition forces that it was considered a threat.
In an indirect reference to Iranian-backed forces that have been gathering in the eastern desert region, a US-led coalition statement cited a recent escalation of tensions and said it would not "tolerate any hostile intent and action of pro-regime forces".
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Russia, also an ally of Syrian President Bashar al-Assad, issued a warning of its own to the United States in response to the downing of the Syrian jet, saying on Monday it would view as targets any planes flying west of the Euphrates River, though it stopped short of saying it would shoot any down.
Still, the Pentagon said it had not seen hostile action from Moscow.
"Public statements aside, we have not seen the Russians do any actions that cause us concern. We continue to operate, making some adjustments for prudent measures," Davis said.
Meanwhile, the US and Russian militaries swapped accusations about an unsafe intercept involving a US spy plane and a Russian fighter jet over the Baltic Sea.
In Syria’s tangled conflict, Washington backs a coalition of rebel forces fighting both President Assad and fighters from the Islamic State of Iraq and the Levant (ISIL, known as ISIS) group.
The US military has repeatedly warned forces fighting on Assad’s side to stay away from a "deconfliction zone", agreed with Russia, near a garrison used by US special forces and US-backed armed groups around Al-Tanf.
In recent weeks, US-led coalition’s warplanes struck pro-Assad government forces to prevent them advancing from the Al-Tanf garrison in southeastern Syria at a spot where the country’s borders meet Iraq and Jordan.
Washington also described those attacks as self-defence.
The competition between the Syrian army and its allies and US-backed rebel groups has stepped up in the Badia desert that stretches to the Iraqi border, after ISIL abandoned large swaths of territory to focus on defending Raqqa and Deir Az Zor.
The Syrian army has since made rapid advances, allowing them to reach the border for the first time in years.