Russia on Monday warned it would track US-led coalition aircraft in Syria as potential "targets" and halted an incident-prevention hotline with Washington after US forces downed a Syrian jet.
The United States moved quickly to contain an escalation, with a top general saying Washington would work to relaunch the "deconfliction" hotline with Russia that was established in 2015.
The downing of the jet and Russia's response came as the US-led coalition and allied fighters battle to oust the Islamic State jihadist group from its Syrian bastion Raqqa.
Analysts say neither Washington nor President Bashar al-Assad's regime appear to be seeking further confrontation, although the risks remain high in Syria's increasingly crowded battlefields and airspace.
Russia's foreign ministry accused Washington of failing to use the hotline before downing the plane near Raqqa, and called for a "careful investigation by the US command" into the incident.
"Any flying objects, including planes and drones of the international coalition, discovered west of the Euphrates River will be tracked as aerial targets by Russia's air defences on and above ground," it warned.
"We will work diplomatically and militarily in the coming hours to re-establish deconfliction," said US General Joe Dunford, chairman of the Joint Chiefs of Staff, referring to the hotline.
Dunford noted that the hotline, which has been vital in protecting both US and Russian forces operating in Syria, remained in use "over the last few hours".
The Syrian jet was shot down on Sunday evening after regime forces engaged the US-backed Syrian Democratic Forces (SDF), an alliance of Arab and Kurdish fighters battling IS, near Raqqa.
An American F/A-18E Super Hornet shot down the Syrian SU-22 as it "dropped bombs near SDF fighters" south of the town of Tabqa, the coalition said in a statement.
Russia's defence minister said the pilot ejected "above IS-controlled territories" and that his fate is unknown.
Earlier Sunday, Syrian troops attacked the SDF near Tabqa, wounding several and chasing them out of town, the coalition said.
It said the warplane was shot down in line with the "rules of engagement".
But Damascus and regime ally Moscow condemned the "aggression".
The Syrian army said the plane was hit while "conducting a mission against the terrorist Islamic State group" and warned of "the grave consequences of this flagrant aggression".
Russia's deputy foreign minister Sergei Ryabkov said it was a "continuation of America's line to disregard the norms of international law".
"What is this if not an act of aggression?"
It was the latest skirmish between the coalition and regime forces in the increasingly tense and crowded space in Syria's north and east.
The coalition backs SDF forces in their months-long bid to capture Raqqa, an operation in which the regime has not been involved.
The SDF entered Raqqa for the first time earlier this month and now holds four neighbourhoods in the east and west of the city.
Syria's army has instead turned its focus further east, to the largely IS-held oil-rich province of Deir Ezzor, where regime forces are besieged in part of the provincial capital.
It is advancing towards the region on three fronts, south of Raqqa, through the Badia desert region in central Syria, and along the eastern border.
But the advances have created conflict with the coalition, particularly along Syria's border, where the US and its allies are training an anti-IS force at the Al-Tanaf garrison.
Earlier this month, the coalition fired on pro-regime ground forces who approached the garrison and shot down a pro-regime armed drone.
Russia halted the hotline only once before, after an April 7 US cruise missile strike on a Syrian regime airbase in retaliation for a suspected chemical weapons attack that killed dozens of civilians.
'Escalation by accident'
Sam Heller, a Syria expert at The Century Foundation think-tank, said the regime was provoking confrontations, but neither side appeared to want a major escalation.
"I think that it was just that the regime engaged in a provocation and then a lower-rung US commander responded in self-defence," he said of Sunday's incident.
"The regime got too close and it got burned."
But provocations by Syria's government and its allies were a potentially risky strategy, he said.
"It doesn't look like anyone currently intends to deliberately escalate further, but when you've got these little skirmishes ... the risk is that you can end up in an escalation by accident."
On Monday morning, the area where the regime and SDF fighters clashed was quiet and the US-backed alliance was continuing to battle IS inside Raqqa, the Syrian Observatory for Human Rights monitor said.
Government forces meanwhile seized the town of Rusafa, south of Raqqa, a key stop on its path to Deir Ezzor and located near provincial oil and gas fields, it added.
Syria's war began in March 2011 with anti-government protests, but has since spiralled into a complex and bloody conflict that has killed more than 320,000 people.
Syria's rebels are now on the back foot after regime advances with support from allies Russia and Iran.
On Sunday, Tehran for the first time fired missiles from its territory against IS positions in Deir Ezzor.
It said the missiles were "in retaliation" for June 7 attacks in Tehran on the parliament complex and shrine of revolutionary leader Ayatollah Ruhollah Khomeini that killed 17 people and was claimed by IS.
On the diplomatic front, a fresh round of Syria peace talks in Kazakhstan's capital Astana has been scheduled for July 4-5, the Kazakh foreign ministry said.