Russia on Saturday accused the United States of "international banditry" after Washington announced its intention to protect Syria's oil fields which are controlled by Kurdish forces.
The statement comes after US Defense Secretary Mark Esper said US troops were reinforcing their positions, including with mechanised forces, in Deir Ezzor, the country's largest oilfields, near the Iraqi border.
Their mission will be to prevent the Islamic State group from gaining access to oil fields and securing "resources that may allow them to strike within the region, to strike Europe, to strike the United States," Esper told reporters on a visit to Brussels.
Some 200 US troops are currently stationed there.
"What Washington is currently doing - seizing and placing under control the oil fields of eastern Syria - is simply international banditry," Russia's defence ministry said in a statement.
It said all hydrocarbon deposits in Syria do not belong to "the Islamic State terrorists" and "even less to US defenders against Islamic State terrorists, but exclusively to the Syrian Arab Republic.