The U.S. military has begun bolstering its troop numbers in a part of eastern Syria where President Donald Trump has said he wants to protect oil fields, U.S. defense officials said Saturday.

The U.S. troops began arriving in Deir al-Zour Province in a convoy from northern Iraq. The defense officials, speaking on condition of anonymity due to the sensitivity of the issue, said the forces will reinforce American troops in coordination with the Kurdish-led Syrian Democratic Forces, who have teamed with the Pentagon on operations against ISIS for years.

The additional forces will help "prevent the oil fields from falling back into the hands of ISIS or other destabilizing actors," one U.S. defense official said. "We will not discuss details or timelines of those forces for security reasons," the official added.

News photographers in the region captured photographs that show a convoy of about a dozen vehicles rolling through the Syrian city of Qamishli, many with American flags flying on them. The majority of the vehicles were mine-resistant armored vehicles, with a few civilian trucks seemingly mixed in.

The oil-field protection plan calls for several hundred U.S. troops to return to Syria but "less than a battalion," U.S. officials said. A battalion in most U.S. units numbers 800 to 1,000 troops.

The convoy Saturday is the latest move in a whirlwind month for the Pentagon in Syria that began with Turkey telling the United States that it would launch an offensive against Kurdish parts of northern Syria. The White House announced Oct. 6 that it would not stand in the way despite years of the Pentagon partnering with Syrian Kurds on operations against ISIS.

Trump decided Oct. 13 to withdraw virtually all 1,000 troops from northern Syria, as the SDF reached an agreement with the Syrian regime for protection. U.S. forces rapidly withdrew from several bases afterward, leaving behind equipment and even using F-15 jets to bomb one of their former headquarters to render it unusable by anyone else.

But Trump was persuaded to move some U.S. troops back into a section of eastern Syria, farther from the border with Turkey. U.S. officials have said that the new mission around the oil fields there will prevent ISIS from capturing them but also allow the Pentagon to continue carrying out counterterrorism operations on the militant group and maintain control of the airspace overhead.

The U.S. presence also will make it more difficult for the Syrian regime or Russian forces aligned with them to seize the oil fields.

In Moscow, Russia's Defense Ministry denounced the U.S. troop movements into eastern Syria on Saturday.

"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," said Defense Ministry spokesman, Maj. Gen. Igor Konashenkov, according to media reports.

Earlier this week, Russia added about 300 military police to its contingent in northern Syria to help patrol a region along the Turkish border.

Russia said the additional battalion of military police comes from Chechnya, a Russian region that saw two devastating separatist wars in the late 1990s and the early 2000s, before Moscow regained control. Troops from Chechnya, known for their fierce warrior spirit, have regularly been sent to Syria in recent years.

The Associated Press contributed to this report.