Russia has threatened to target areas in Syria where American special forces and US-backed militia are operating if its own forces came under fire from them - something it said had already happened twice.
The country's defence ministry said Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias fighting, had taken up positions on the eastern banks of the Euphrates alongside US special forces.
The SDF had twice opened fire with mortars and artillery on Syrian troops who were working alongside Russian special forces, it claimed.
Major-General Igor Konashenkov said in a statement that a representative of the US military command in Al Udeid, the American operations centre in Qatar, had been "told in no uncertain terms that any attempts to open fire from areas where SDF fighters are located would be quickly shut down."
He added: "Fire points in those areas will be immediately suppressed with all military means."
Moscow also claimed that the SDF had diverted from the battle to take the northern city Raqqa to the province of Deir al-Zor, around 100 miles to the east, where Russian special forces are helping the Syrian army push out Isis militants.
In Deir al-Zor, the Islamic militants are battling two separate offensives, launched by the SDF on one side and the Syrian army and its allies on the other.
The Syrian army, backed by Russian and Syrian war planes, has captured about 62 miles of the west bank of the Euphrates this month, reaching the Raqqa provincial border earlier this week, the Syrian Observatory for Human Rights reported.
Syrian troops also crossed to the eastern side of the river on Monday. The SDF's advances have been on the eastern bank of the river.
The convergence of the two rival offensives has increased tensions in Deir al-Zor. The US-backed militia said over the weekend that they had come under attack from Russian jets and Syrian government forces, something Moscow denied.
The SDF later warned against any further Syrian army advances on the eastern riverbank.
Russia's Defence Ministry subsequently said the waters of the Euphrates had risen as soon as the Syrian army began crossing it, suggesting this could only have happened if upstream dams held by the U.S.-backed opposition had been opened.
Major General Konashenkov, in his statement, questioned the nature of SDF's relationship with Issis. Russian drones did not detect any clashes between the two groups when SDF fighters approached Deir al-Zor, he said.
