Russia accuses US of 'banditry' in guarding Syria's oil
The statement comes after US Defence Secretary Mark Esper said US troops were reinforcing their positions, including with mechanised forces, in Deir az-Zour, 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."
Washington's U-turn came after US President Donald Trump earlier this month promised to withdraw the country's troops from northeastern Syria, a controversial move that allowed Turkey to launch its long-hoped-for offensive against the Syrian Kurdish forces unimpeded.
Trump has repeatedly flip-flopped over the withdrawal decision over the past few weeks, at first being seen to have given Turkey the go-ahead for its invasion and then pledging sanctions against Ankara, on which he later went back.
Critics of the move had warned that a Turkish invasion could embolden IS sleeper cells in the region and facilitate mass jail breaks of imprisoned members of the extremist group, strengthening its numbers and possibly enabling it to recapture swathes of territory in Syria.
Trump is now apparently heeding those warnings - although critics of the US say the pledge to secure Syrian oil fields is an example of "oil imperialism".
Adding armoured reinforcements to the area could mean sending several hundred more US troops, the Associated Press reported.
Read more: Syria Weekly: Trump U-turn could see US troops guarding Syrian oil fields
A similar number are currently in the process of being withdrawn from an area closer to Turkey's border where Russian troops are currently filling the vacuum, as part of a deal to enforce a buffer zone free of the Kurdish forces.
Speaking seperately on Friday, Trump said he was "getting our troops out" of Syria, without mentioning Esper's announcement.
"We are doing well in Syria, with Turkey and everybody else that we're dealing with," the president said. "We have secured the oil... We have a couple of people that came knocking, we said don't knock. And I think I would say that things are going very well."
White House officials would not clarify whom he was referring to as "knocking."
The US special envoy for Syria, James Jeffrey, said on Friday he had spoken with a Russian official about an unspecified issue in Syria's oil region.
"We are currently very concerned about certain developments in the south, in the Deir az-Zour area," he said. "I've talked to my Russian colleague about that and we're having other contacts with the Russians concerning that situation. We think it is under control now."
Asked about America's shifting Syria strategy, Esper said the US mission has always been to prevent the resurgence of IS.
"That mission remains unchanged," he said. "If IS has access to the resources, and therefore the means to procure arms or to buy fighters or whatever else they do, then it means it makes it more difficult to defeat IS."