The US military said it has shot down an Iranian-made armed drone in southern Syria, marking the third time this month that the US has downed aircraft affiliated with Syrian president Bashar Assad's government.
The US-led coalition fighting Islamic State said a US F-15 fighter jet shot down the drone "after it displayed hostile intent" while approaching a military camp near the Syria-Jordan border.
A similar drone was shot down in the same location after it dropped munitions near coalition forces on June 8.
The repeated incidents in the vicinity of the Tanf camp, where US forces train and advise local ground forces in the fight against IS, add to soaring regional tensions that could spiral out of control just as the fight against the extremists enters a crucial phase, with US-backed forces pushing into the group's de facto capital, the Syrian city of Raqqa.
The US on Sunday shot down a Syrian jet near Raqqa after it dropped bombs near the US-allied Syrian Democratic Forces, which are battling IS.
Russia condemned the US action and in retaliation suspended a hotline intended to prevent such incidents.
The US has also fired on Syrian government ground forces in the east on two occasions in just the last month.
A defence official said the drone shot down on Tuesday was a Shaheed 129 and appeared to have been operated by "pro-regime" forces.
There are concerns that further confrontations could lead to a breakout of hostilities that would hinder the battle against IS.
Australia on Tuesday suspended its air strikes against IS targets in Syria as a precaution, after Russia announced it was suspending the hotline and warned the US-led coalition not to fly over Syrian army positions west of the Euphrates River.
In another first, Iran - another close Assad ally - fired ballistic missiles at IS targets in eastern Syria, in the province of Deir el-Zour, later on Sunday.
Iran's Revolutionary Guard described the strike as revenge for IS attacks on Tehran earlier this month that killed at least 18 people and wounded more than 50.
A spokesman for the Guard, General Ramazan Sharif, said all six ballistic missiles hit their targets, citing "local sources" and drone footage.
Gen Sharif said the missile launch reflected Iran's "military power", though Iran has no intention of starting another war.
It is still unclear what the missiles struck, and Iran has provided few details.
Israeli security officials said on Monday they were studying the missile strike to see what they could learn about its accuracy and capabilities.
Fighting and bombardment resumed early on Tuesday between Syrian government forces and rebels in the southern city of Daraa and nearby areas, where a 48-hour truce had gone into effect on Saturday.
The truce was meant to be extended but now appears to have collapsed.
A Syrian military official in Damascus said the truce was not extended.
"The situation is back as it was in Daraa city and fighting is ongoing," said Daraa-based opposition activist Ahmad al-Masalmeh. "The regime made the truce and then violated it."
The activist-run Syrian Observatory for Human Rights reported that government troops captured a hill west of the city and a former army base that was held by rebels.
It said government helicopters dropped 55 barrel bombs on Daraa while warplanes carried out about 20 air strikes.
Daraa is where the March 2011 uprising against the Assad family's four-decade rule began.
The conflict quickly descended into a civil war, which has since left at least 400,000 people dead.
The push by Syrian government forces in the Daraa area appears headed for the Jordanian border, two weeks after Assad's troops reached the border with Iraq for the first time in years.