A Pentagon official said Thursday that the U.S. is planning to send additional troops into northeastern Syria to protect oil fields from ISIS. The announcement comes as Russia, which has gained new power in the region, ordered all U.S. troops out of the country and called the remaining American troops an "occupying force."

The announcement is a reversal of President Trump's decision to pull U.S. forces from the area, which sparked a Turkish cross-border offensive earlier this month.

While most U.S. troops are withdrawing from Syria, the Pentagon is planning a major increase in firepower to protect the ones left behind. If approved, a combat unit armed with tanks would be sent into an area along the Euphrates River to reinforce about 200 lightly armed troops who are staying in Syria to protect the oil fields.

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"One of the most significant gains by the U.S. and our partners in the fight against ISIS was gaining control of oil fields in Eastern Syria — a crucial source of revenue for ISIS," a defense official said.

A look at where U.S. military assets would be deployed to assist troops protecting oil fields from being recaptured by ISIS. CBS News

Republican Senator Lindsey Graham said he was briefed on the situation Thursday by Joint Chiefs Chairman General Mark Milley.

"There's a plan coming together from the Joint Chiefs, that I think may work, that may give us what we need to prevent ISIS from coming back around," he said.

Mr. Trump's decision to order 1,000 U.S. troops out from northern Syria more than a week ago was met with widespread condemnation by Democrats and Republicans in Congress, who accused the president of abandoning the Kurdish allies who were instrumental in defeating ISIS. The Kurds were also in control of several prisons holding ISIS fighters and their families.

After Mr. Trump announced the U.S.' withdrawal, Turkey launched an assault on the Kurds. The United Nation's deputy humanitarian chief, Ursula Mueller, has said almost 180,000 people, including 80,000 children, fled their homes in northeastern Syria because of the offensive.

On Wednesday, the president announced the U.S. would lift sanctions on Turkey because of a "permanent" cease-fire in the conflict between Turkey and the Kurds. The Kurds largely withdrew from the region under the terms of a 120-hour cease-fire negotiated by the U.S.

That announcement came after Turkish President Recep Tayyip Erdogan and Russian President Vladimir Putin brokered a new six-day cease-fire, which allowed Russia to step into the power vacuum created after U.S. troop withdrawal.

Mr. Trump has lauded the ceasefire deal as "a great outcome," and praised Erdogan.

"We have done them a great service and we've done a great job for all of them, and now we're getting out," Mr. Trump said Wednesday. "Let someone else fight over this long blood-stained sand."

David Martin contributed to this report.