KABUL (Reuters) - The United States is considering keeping some troops in northeastern Syria to stop oil falling into the hands of Islamic State or others but no decision has yet been made, Defense Secretary Mark Esper said.

President Donald Trump decided earlier this month to withdraw all 1,000 U.S. troops from the region, a move widely criticized as a betrayal of Kurdish allies who had fought for years alongside them against Islamic State.

U.S. troops crossed into Iraq early on Monday as part of the withdrawal process. Trump began pulling U.S. troops back from northeastern Syria in early October, opening the way for Turkish troops to launch an offensive against the Kurdish fighters.

“We have troops in towns in northeast Syria that are located next to the oil fields, the troops in those towns are not in the present phase of withdrawal,” Esper told reporters during a visit to Afghanistan.

“The purpose is to deny access, specifically revenue to ISIS (Islamic State) and any other groups that may want to seek that revenue to enable their own malign activities,” he said.

Esper said there had been discussions about keeping some of the U.S. troops, who were with the Syrian Democratic Forces (SDF), in place. He said he had not presented that option yet, but the Pentagon’s job was to look at different options.

The SDF, spearheaded by the Kurdish YPG militia, has been the main partner for the U.S.-led coalition in Syria.

“There has been a discussion about possibly doing it (keeping some troops), there has been no decision with regard to numbers or anything like that,” Esper added.

A Reuters cameraman saw more than 100 vehicles carrying U.S. troops crossing from the northeast tip of Syria, where Turkey has agreed to pause its offensive for five days under a deal agreed between Washington and Ankara. The truce expires late on Tuesday.

Turkish President Tayyip Erdogan has warned that Ankara will resume its military assault in Syria when the deadline expires if the SDF have not pulled back from its proposed “safe zone” area spanning the border.

Ankara considers the YPG a terrorist group because of its links to Kurdish insurgents in southeast Turkey.