The warning came amid rising tensions in the Syrian desert between the United States and its Kurdish and Arab allies on the one hand, and Russia, the Syrian regime and Iranian-backed militias on the other, as both converge on Islamic State-held territory in eastern Syria.
A Russian military spokesman, Maj. Gen. Igor Konashenkov, said the U.S.-backed Syrian Democratic Forces, or SDF, had twice in recent days shelled Syrian government positions outside Deir al-Zour, a strategic city in the region.
Konashenkov said Russian special forces are helping government troops fight Islamic State militants in the battle for the city.
Moscow has conveyed to the U.S. military command "in no uncertain terms that any attempts to open fire from areas where SDF fighters are located would be quickly shut down," Konashenkov said in a statement. "Firing positions in those areas will be immediately suppressed with all military means."
The tensions have been escalating as the SDF advances through Islamic State-held territory from the northeast toward Russian-backed Syrian government forces advancing from the west, risking a collision at some point.
The warning that Russia is prepared to take military action to check any further advances by the U.S.-led coalition came after the United States on Saturday said Russian warplanes had struck an SDF position north of Deir al-Zour. Soldiers of the U.S.-led multinational coalition were present at the time of the strike, according to a U.S. military statement.
The United States says it maintains about 500 troops in northern and eastern Syria, mainly Special Operations forces advising the SDF. But the actual number is larger, because the publicly announced figure does not include service members assigned to Syria for less than 18 months, according to Col. Ryan Dillon, a U.S. military spokesman.
But there is currently no such agreement defining the U.S. and Russian areas of operation around the key towns and villages stretching south along the Euphrates River from Deir al-Zour toward the town of Bukamal on the Iraqi border. The area contains most of Syria's oil and controls access to the Iraqi border, and is viewed as a critical prize for all sides involved.
Syrian government officials have said they regard it as essential to take control over the area, in pursuit of their goal of restoring sovereignty over all of Syria. They also want to prevent the United States from gaining influence over any more Syrian territory.
For Iran - a key backer of Syrian President Bashar al-Assad - the Iraqi border area represents an opportunity to cement its arc of influence stretching from Tehran through Baghdad and Damascus to Beirut on the Mediterranean.
U.S. military officials say their primary goal is to defeat the Islamic State, which is believed to have concentrated many of its senior leaders in the area and is expected to make its last stand along the Euphrates River valley.
But Trump administration officials have also said on several occasions that they have set the additional a goal of containing any further expansion of Iranian influence in areas where the Islamic State is defeated. The Russian threat appeared to serve a warning that if Washington intends to take on Iran in the area, it will have to contend with Russia as well.
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