Russia warns that it will treat all US coalition aircraft west of the Euphrates River in Syria as "airborne targets" after a US fighter jet shot down a Syrian warplane.
Russia's Defence Ministry announced that it was ending US-Russian co-operation in Syria that had been designed to prevent a direct conflict between the great powers.
"Repeated military actions by US aircraft against the lawful armed forces of a United Nations member state, under the guise of a 'fight against terrorism,' are a profound violation of international law and, in fact, military aggression against the Syrian Arab Republic," the Russian Defence Ministry said in a statement.
"As a result of the strike, the Syrian plane was destroyed. The Syrian pilot catapulted into an area controlled by Islamic State terrorists. His fate is unknown."
The US military confirmed a US F-18 Super Hornet shot down a Syrian SU-22 on Sunday, after it dropped bombs near the US partner forces known as the Syrian Democratic Forces. SDF fighters are aligned with the Americans in the campaign against the Islamic State group.
Russia has been a staunch supporter of Syria's beleaguered President Bashar Assad and has been providing an air cover for his offensive against IS since 2015.
Earlier Russian Deputy Foreign Minister Sergei Ryabkov compared the downing to "helping the terrorists that the US is fighting against."
"What is this, if not an act of aggression," he asked.
Meanwhile, the US-backed opposition fighters said Assad's forces have been attacking their positions in the northern province of Raqqa and warned that if such attacks continue, the fighters will take action.
Clashes between Syrian troops and SDF would escalate tensions in the country and open a new frontline in the many complex battlefields of the civil war, now in its seventh year. Clashes between the Kurdish-led SDF and Syrian forces have been rare and some rebel groups have even accused them of coordinating on the battlefield.
The clashes come as both sides are fighting against Islamic State, with SDF fighters now focusing on their march into the northern city of Raqqa, the de facto capital of IS.
Government forces have also been attacking IS in northern, central and southern Syria, seizing 25,000 square kilometres and reaching the Iraqi border for the first time in years.
SDF spokesman Talal Sillo said the government aims to thwart the SDF offensive to capture the city of Raqqa.
He said government forces began attacking SDF on Saturday, using warplanes, artillery and tanks in areas that SDF had liberated from IS.