Secretary of Defense Mark Esper said at a press conference on Monday that American mechanized forces have already deployed to protect oil fields in eastern Syria. The mission, on its surface, aims to keep these oil fields in the hands of the Kurdish-led Syrian Democratic Forces, Washington’s former allies in the fight against IS.

The American troops, he said, will “respond with overwhelming military force against any group who threatens the safety of our forces there.” When asked whether the US would potentially respond with force against Russian or Syrian forces, Esper simply responded “yes.”

Esper’s apparent threat comes hot on heels of Moscow accusing the US of facilitating the oil smuggling in Syria. Russian Defense Ministry spokesperson Major General Igor Konashenkov called the US oilfield operation “state-sponsored banditry,” and accused Washington of leaving its forces there to help oil smugglers pillage Syria’s national resources. The ministry published aerial images on Saturday, that it claims show crude oil being smuggled out of the country “under the strong protection of the US.”

Though much noise has been made about protecting the oil fields from the remnants of IS, the shattered jihadist group has not made any moves towards seizing them. Asked whether the fields and the American forces there were under any threat, Esper replied “not at this time.”