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 images showing a convoy of about a dozen Army vehicles rolling past oil pumps near the Syrian city of zamishli, many with American flags flying on them. The majority of the vehicles were mine-resistant armored vehicles, with a few civilian trucks mixed in. No tanks or Bradley Fighting Vehicles appear to have moved into Syria, although U.S. officials have said they are considering both options in the coming days.

The oil-field protection plan calls for several hundreds of 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.

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 quickly withdrew from several bases afterwards, leaving behind some equipment and even using the 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 stretching from Hasakah south to Deir al-Sour, far from the border with Turkey. U. S. officials have said that the new mission around the oil fields there will prevent the Islamic State 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 will also make it more difficult for the Syrian regime or Russian forces aligned with them from seizing the oil fields.

In Moscow, the Russian Defense Ministry denounced the U.S. movements troop ed in eastern Syria on Saturday. Earlier this week, Russia added about 300 military police to its content in northern Syria to help patrol the region along the Turkish border.

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, according to media reports.

Brian Murphy in Washington contributed to this report.