The American military has begun crossing over into northwestern Syria from Iraq in order to secure oil fields that are at risk of falling to ISIS, NBC News reports. A convoy of lightly armored vehicles was seen crossing into the Deir ez-Zor region of Syria, where the U.S. has already partnered with the Kurdish-led Syrian Democratic Forces. The American force is likely to be fewer than 400 troops, down from the 1,000 stationed at Deir ez-Zor and another base, al-Tanf, before the U.S. withdrawal, according to an official familiar with Syria planning who spoke to The Daily Beast.

The move comes two days after President Trump was reportedly briefed by Defense Secretary Mark Esper and chairman of the Joint Chiefs of Staff, Gen. Mark Milley, on the importance of securing the oil fields. Trump has said that safeguarding these resources is why the U.S. has not fully withdrawn from Syria—a plan he abruptly announced earlier this month. The president has previously claimed that the U.S. eradicated ISIS, but the terrorist group may have reemerged after Trump’s withdrawal unleashed chaos in the region.