AMMAN (Reuters) - The U.S.-led coalition said on Tuesday it had shot down an armed "pro-Syrian regime" drone that had been bearing down on its forces near a garrison in Syria's southeast, and a Western intelligence source identified the aircraft as Iranian.
It marked the second time in three days U.S. forces have shot down an aircraft operated by Damascus or its allies in Syria, and reflected mounting tensions over a stretch of the Syrian-Iraqi frontier where U.S. forces have established a base.
In a statement, U.S. forces said the drone was fired on after it "displayed hostile intent and advanced on coalition forces". The 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 coalition statement 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, the coalition statement cited a recent escalation of tensions and said it would not "tolerate any hostile intent and action of pro-regime forces".
Tensions escalated on Sunday as the U.S. army brought down the jet near Raqqa and Iran launched missiles at Islamic State targets in eastern Syria - the first time each state has carried out such actions in the multi-sided Syrian war.
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.
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 adavances allowing them to reach the border for the first time in years.
(Reporting by Suleiman Al-Khalidi; Editing by Richard Balmforth)