The U.S. Defense Department said on Thursday it planned to beef up its presence in the northeast corner of Syria to protect oil fields from being retaken by a potentially resurgent ISIL. 

"One of the most significant gains by the U.S. and our partners in the fight against ISIS (ISIL) was gaining control of oil fields in Eastern Syria – a crucial source of revenue for ISIS (ISIL)," a defense official said in a statement. 

"The U.S. is committed to reinforcing our position, in coordination with our SDF partners, in northeast Syria with additional military assets to prevent those oil fields from falling back into the hands of ISIS (ISIL) or other destabilizing actors," the official said on grounds of anonymity. 

"We must deny ISIS (ISIL) this revenue stream to ensure there's no resurgence." 

President Donald Trump announced last week that the U.S. would pull all of its troops out from northern Syria, where they had served as a buffer between the Kurdish-led Syrian Democratic Forces (SDF), government forces and Turkish troops.

But on Wednesday, Trump said a few U.S. troops would be staying behind to protect 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," Trump said.

Meanwhile the SDF said in its statement on Thursday that Turkish forces had attacked three villages "outside the area of the ceasefire process," forcing thousands of civilians to flee. 

"Despite our forces' commitment to the ceasefire decision and the withdrawal of our forces from the entire ceasefire area, the Turkish state and the terrorist factions allied to it are still violating the ceasefire process," it said. 

"Our forces are still clashing," it said, urging the United States to intervene to halt the renewed fighting.

Turkey's Defense Ministry did not comment directly on the SDF report but said five of its military personnel had been wounded in an attack by the YPG militia around the border town of Ras al Ain, near where the three villages are located. 

Turkey previously said it reserved the right to self-defense against any militants who remain in the area despite the truce, a pledge repeated by President Recep Tayyip Erdogan on Thursday.