Russia on Monday warned it would track US-led coalition aircraft in Syria as potential "targets" and halted a military hotline with Washington after US forces downed a Syrian jet.
The United States moved quickly to contain an escalation, with a top general saying it would work to relaunch the "deconfliction" hotline 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 (IS) jihadist group from its Syrian bastion Raqa.
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.
"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," said Russia's foreign ministry.
It said Washington had failed to use the hotline -- a vital incident-prevention tool -- before downing the plane near Raqa.
"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.
Pentagon spokesman, Major Adian Rankine Galloway, said the US had taken "prudent measures to re-position aircraft over Syria" to ensure the safety of pilots.
The jet was downed on Sunday evening after regime forces targeted the US-backed Syrian Democratic Forces, an alliance of Arab and Kurdish fighters battling IS.
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, said the US-led coalition.
Russia's defence minister said the pilot ejected "above IS-controlled territories" and that his fate was unknown.
- 'Flagrant aggression' -
The coalition said Syrian troops attacked SDF fighters near Tabqa, south of Raqa, wounding several and chasing them out of the town.
It said the warplane was targeted in line with the "rules of engagement".
Damascus and its ally Moscow condemned the "aggression".
The Syrian army said the plane was hit while on a mission against IS jihadists 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.
Backed by the coalition, SDF forces entered Raqa for the first time two weeks ago, after months of fierce fighting, and have seized four neighbourhoods.
The Syrian army is not involved in the offensive, and has instead turned its focus further east to the largely IS-held, oil-rich province of Deir Ezzor.
Pro-regime forces are moving on three fronts, south of Raqa, through the Badia desert region in central Syria, and along the eastern border.
The advance have created tensions, 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 threatened to scrap the hotline 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.
"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 Raqa, said the Syrian Observatory for Human Rights.
Government forces meanwhile seized the town of Rusafa, south of Raqa, a key stop on its path to Deir Ezzor and located near provincial oil and gas fields, the monitor added.
Syria's war began in March 2011 with anti-government protests that spiralled into a complex and devastating conflict that has killed more than 320,000 people.
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 Monday, the United Nations expressed deep concern over the risk of escalation in Syria.
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