Russia said on Monday it would 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.
The statement from the Russian Defence Ministry stopped short of threatening to attack such US aircraft.
However, the ministry also announced that it was ending cooperation on a US-Russian agreement that had been designed to prevent a direct conflict between the two powers in Syria.
"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 an emailed statement.
The Defence Ministry said the Syrian plane was destroyed and the fate of the pilot unknown.
Russia, which entered the Syrian civil war in 2015 to support the beleaguered regime of Syrian President Bashar al-Assad, a long-time ally, has long claimed that the US involvement in the war without the regime's permission violates international law.
The US, which provides air support to rebel forces on the ground battling the Islamic State, has accused al-Assad's regime of committing crimes against humanity, including killing civilians.
The US said that on Sunday a US F/A-18E Super Hornet fighter jet shot down a Syrian regime Su-22.
It was the first time in the six-year-old war that the US downed a Syrian jet, sparking fears of wider escalation between opposing sides.
The Su-22, a Russian-made warplane, had attacked a town held by the US coalition-aligned Syrian Democratic Forces, "wounding a number of SDF fighters and driving the SDF from the town," the US coalition's press service told dpa.
The US coalition, whose mission is to defeat the terrorist group Islamic State, "does not seek to fight Syrian regime, Russian, or pro-regime forces partnered with them, but will not hesitate to defend coalition or partner forces from any threat," the US coalition said in emailed comments.
Meanwhile, top Russian diplomat Sergei Lavrov said that the next Syrian peace talks in the neutral Central Asian state of Kazakhstan will take place on July 10.
UN special envoy Staffan de Mistura is expected to attend the round of talks, Lavrov said, according to comments carried by Russian state media.
In Syria, US-backed Syrian militias vowed to retaliate to any attack by government troops on their positions south-west of the city of al-Raqqa, the de-facto capital of Islamic State militia.
SDF-led fighters are pressing ahead with a campaign to capture al-Raqqa, which has been under control of Islamic State radical group since 2014.
"The aim of repeated regime attacks against our forces in the past few days is to abort the process of liberating the city of al-Raqqa," Syrian Democratic Forces (SDF) spokesman Talal Silo said in a statement.
Regime attacks "will force us to retaliate and use our legitimate right to defend our forces," Silo added.
The Syrian army said the downed plane was on a combat mission against the extremist militia in al-Raqqa.
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