The U.S. military said it had shot down an armed, Iranian-made drone that had been bearing down on its forces near a garrison in Syria's southeast on Tuesday, in the latest sign of increasingly frequent confrontation with Damascus and its allies.
The incident closely followed Sunday's U.S. downing of a piloted Syrian army jet in the southern Raqqa countryside after it dropped bombs near U.S.-backed forces.
That was a rarity in modern warfare, representing the first time the U.S. Air Force had downed a manned jet since 1999.
In the latest incident, the Pentagon said a U.S. F-15 aircraft flying over Syrian territory fired on the drone after it displayed hostile intent and advanced on coalition forces.
Pentagon spokesman Navy 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 specifically was operating it.
A Western intelligence source said it was "unquestionably Iranian".
"They are testing the limits," the source told Reuters without elaborating.
The area falls in a part of Syria that was recently identified as a military priority by Damascus, and is seen as strategically important for Iran as it seeks to secure a land corridor between forces it backs in Syria and Iraq.
The U.S.-led coalition said the location was close to where another "pro-regime" drone, which intelligence sources had also identified as Iranian, was shot down on June 8 after dropping bombs near coalition forces.
In an indirect reference to Iranian-backed forces that have been gathering in the eastern desert region, a U.S.-led coalition statement cited a recent escalation of tensions and said it would not "tolerate any hostile intent and action of pro-regime forces".
Russia, like Iran an ally of 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 by 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 U.S. and Russian militaries swapped accusations about an unsafe intercept involving a U.S. 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 Bashar al-Assad and Islamist militants, while Assad is backed by Russia, Iran and Shi'ite militia.
The U.S. 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 U.S. special forces and U.S.-backed militia around Al Tanf.
On several occasions in recent weeks, warplanes of the U.S.-led coalition have also struck pro-government forces to prevent them advancing from the Al Tanf garrison in southeastern Syria at a spot where the country's borders join Iraq and Jordan.
Washington also described those strikes as self-defense.
The competition between the Syrian army and the militias and U.S.-backed Free Syrian Army (FSA) rebels has stepped up in the Badia desert that stretches to the Iraqi border after Islamic State abandoned large swathes of territory as it defends Raqqa and Deir Zor.
The Syrian army has been able to make rapid advances allowing them to reach the border for the first time in years.