U.S. forces have been ordered to prepare to provide security for oil fields in eastern Syria to prevent them from falling into the hands of the Islamic State militant group, a defense official told NBC News on Thursday.

The possible number of personnel who could be involved in security reinforcement at oil fields in the Deir ez-Zor region hadn't been determined, but "we're not talking thousands," the official said.

The operation, if approved, would be conducted alongside the mostly Kurdish Syrian Democratic Forces, the official said.

Another defense official said that the United States was committed to reinforcing its position, in coordination with SDF partners, in northeast Syria with additional military assets to prevent those fields from falling back into the hands of ISIS or others. The U.S. must deny ISIS this revenue stream to ensure it doesn't resurge, the officials said.

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A senior defense official separately told NBC News that President Donald Trump was briefed by Defense Secretary Mark Esper and Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, on the importance of securing the oil fields so they can't be seized and used to fund ISIS' terrorist activities, as were Senate Republicans during a Situation Room briefing.

The Oil Fields discussed in my speech on Turkey/Kurds yesterday were held by ISIS until the United States took them over with the help of the Kurds. We will NEVER let a reconstituted ISIS have those fields! — Donald J. Trump (@realDonaldTrump) October 24, 2019

The official said Marine Gen. Kenneth McKenzie, the head of U.S. Central Command, was being given a number of options, among them sending tanks to the region, deploying forces pulled from an armored brigade. An additional consideration is the possible use of air assets to support ground forces.

Trump signaled the possible action in an address from the White House on Wednesday when he said, "We're going to be protecting" Syrian oil fields.

"We have secured the oil, and, therefore, a small number of U.S. troops will remain in the area where they have the oil," he said.

The U.S. announced earlier this month that it would withdraw its forces from northern Syria. The move was seen as a major blow to the Kurdish-led forces.

Esper said at a press conference in Brussels Friday that the U.S. will have a reduced presence in an effort to deny ISIS access to oil revenue, "as a re-position for the next phase of the 'Defeat ISIS' campaign."

"We are now taking some actions — I'm not going to get into the details — to strengthen our position at Dar Azar to ensure that we can deny ISIS access to the oil fields, because we want to make sure that they don't have access to the resources that may allow them to strike within the region, to strike Europe, to strike the United States," Esper said.

"Otherwise, all the other forces are intended to return home," he said.

Turkey and the U.S. are NATO allies, while the Kurds have been crucial U.S. allies in the war against ISIS.

But Turkey considers the Kurdish forces to be an enemy, and soon after the U.S. announcement, it began an offensive inside Syria in the Kurdish held area, leading critics to accuse the U.S. of having abandoned its longtime partner.