Russia has warned it will treat American and British jets operating in parts of Syria where its air forces are present as “targets”, after the US shot down a Syrian warplane for the first time in the six-year-war.
Moscow, Syria's main ally, announced that it would start tracking jets and drones of the US-led coalition and halt an incident-prevention hotline with America, which was set up to avoid accidents in the crowded skies above Syria.
"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," the defence ministry warned.
On Monday Sean Spicer, Donald Trump's spokesman, insisted the US would not back down and would "do what we can to protect our interests."
Mr Spicer added: "The escalation of hostilities among the many factions that are operating in this region doesn't help anybody. And the Syrian regime and others in the regime need to understand that we will retain the right of self-defence, of coalition forces aligned against ISIL"
The Syrian SU-22 fighter bomber was shot down by an American F18 Super Hornet on Sunday after it had dropped bombs near the US-backed Syrian Democratic Forces (SDF) north of the Islamic State of Iraq and the Levant (Isil)-held city of Raqqa.
The US, which has special forces troops in the area, had earlier sent a warning to the Syrian military to stop targeting the forces and called on Russia to rein in its ally, but they were ignored.
Russia later denied that it was informed of the planned strike.
It is believed to be the first air-to-air kill of a manned aircraft by a US military jet since the Kosovo campaign in 1999 and could draw the US further into the intractable conflict.
Russia, which intervened militarily to back the Syrian regime in 2015, on Monday condemned the US action, saying it flouted international law.
"It is, if you like, help to those terrorists that the US is fighting against, declaring they are carrying out an anti-terrorism policy," Sergei Ryabkov, Russia’s deputy foreign minister, said, adding it was a “dangerous escalation”.
Konstantin Kosachev, chairman of the international affairs committee in Russia’s upper house of parliament, said: “It is hard for me to choose any other words but these: if you [the US] can’t help you should at least not interfere. As your ‘efforts’ once again do nothing but help the militants.
“You are fighting the wrong party: it is not the Syrian army that perpetrates terror attacks in European capital cities.”
The SDF meanwhile said it would not allow further attacks on its troops, a combination of Kurds and Arabs which are leading the fight for Isil’s self-styled capital Raqqa.
"The regime's forces...have mounted large-scale attacks using planes, artillery, and tanks," Talal Selo, Syrian Democratic Forces (SDF) spokesman, said in a statement. "If the regime continues attacking our positions in Raqqa province, we will be forced to retaliate...and defend our forces."
The coalition does “not seek to fight the Syrian regime, Russian or pro-regime forces” but will not “hesitate to defend itself or its “partnered forces from any threat”, a Pentagon statement said.
The US-led coalition has in recent weeks escalated its aerial bombing campaign in northern Syria and Raqqa province. US-backed forces have encircled the city of Raqqa and captured several districts from the militants.
The Syrian regime are not fighting in the battle for Raqqa, but its troops have been edging closer to Isil-controlled areas of eastern Syria.
On several occasions in recent weeks, warplanes of the US-led coalition have also struck pro-government forces to prevent them advancing from a US-controlled garrison in southeastern Syria.
Top White House officials have claimed that the Trump administration is pushing to broaden the war in Syria, viewing it as an opportunity to confront Iran, which backs President Bashar al-Assad, and its proxy forces on the ground there.
Before the administration's targeting of al-Shayrat air base, in reaction to the regime’s chemical attack in April, Washington had previously not engaged the Syrian regime.
America has been flexing its muscles in Syria ever since President Donald Trump came to power, with a pledge to give more power to the Pentagon.
He strongly criticised his predecessor, Barack Obama, for his inability to resolve the Syrian civil war – even blaming Mr Obama for creating the Islamic State of Iraq and the Levant (Isil).