MOSCOW -- Russia's Defence Ministry on Saturday harshly criticized the United States decision to send armoured vehicles and combat troops into eastern Syria to protect oil fields.

U.S. Defence Secretary Mark Esper says the move is aimed at keeping the fields from potentially falling into the hands of Islamic State militants.

On Saturday, a U.S. convoy of over a dozen vehicles was seen driving south of the northeastern city of Qamishli, likely heading to the oil-rich Deir el-Zour area or another base before it. The Syrian Observatory for Human Rights also reported the convoy, saying it arrived earlier from Iraq.

Russian ministry spokesman Maj. Gen. Igor Konashenkov said "what Washington is doing now, the seizure and control of oil fields in eastern Syria under its armed control, is, quite simply, international state banditry."

He added in a statement that "all hydrocarbon deposits and other minerals located on the territory of Syria do not belong to the IS terrorists, and even less to the 'American defenders from IS terrorists,' but exclusively to the Syrian Arab Republic."

"The real cause of this illegal action by the United States in Syria lies far from the ideals that Washington has proclaimed and from the slogans of fighting terrorism," Konashenkov said.

Esper's comments were the latest sign that extracting the U.S. military from Syria is more uncertain and complicated than President Donald Trump is making it out to be.

Earlier this month, Trump ordered U.S. troops out of northeastern Syria, largely turning his back on Syrian Kurds who battled IS alongside the U.S.-led coalition since 2015. But Trump said he will leave up to 300 troops in southern Syria before Esper said another residual force was being considered in southeastern Syria to protect oil fields.

Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State Mike Pompeo spoke by telephone Saturday about Syria.

"From the Russian side, the necessity was emphasized of refraining from steps undermining the sovereignty and territorial integrity of that county," the ministry said in a statement.