The United States will repel any attempt to take Syria’s oil fields away from U.S.-backed Syrian militia with “overwhelming force,” whether the opponent is Islamic State or even forces backed by Russia or Syria, the Pentagon said on Monday.

The U.S. military announced last week it was reinforcing its position in Syria with additional assets, including mechanized forces, to prevent oilfields from being taken over by remnants of the Islamic State militant group or others.

U.S. Defense Secretary Mark Esper offered some of his most detailed remarks to date about the mission at a news briefing on Monday.

“U.S. troops will remain positioned in this strategic area to deny ISIS access those vital resources. And we will respond with overwhelming military force against any group that threatens the safety of our forces there,” Esper told reporters at the Pentagon.

Pressed on whether the U.S. military mission included denying any Russian or Syrian government forces access to the oilfields, Esper said: “The short answer is, yes, it presently does.”

He noted that the U.S.-backed Syrian Democratic Forces, or SDF, relied on that oil income to fund its fighters, including the ones guarding prisons that hold captured Islamic State fighters.

“We want to make sure that SDF does have access to resources in order to guard the prisons, in order to arm their own troops, in order to assist us with the defeat-ISIS mission,” he said, using an acronym for Islamic State.

“So that’s our mission, is to secure the oilfields.”

President Donald Trump has softened his pullout plans for Syria after a backlash from Congress, including among key Republicans who say he cleared the way for a long-threatened Turkish incursion against the SDF, which had been America’s top ally in the battle against Islamic State. Amid concerns that Islamic State could stage a resurgence in the ensuing power vacuum, Trump said last week a small number of U.S. troops would remain in the area of Syria “where they have the oil,” a reference to oilfields in the Kurdish-controlled region.

That plan for Syria appears unaltered by the U.S. raid on Saturday that led to the death of Islamic State leader Abu Bakr al-Baghdadi.