The United States will keep about 600 troops in Syria, AFP reported referring to Pentagon chief Mark Esper. 

"We're still moving troops out of northeastern Syria," the US defense secretary said onboard a plane as he travelled to Seoul, where he begins a tour of Asia on Thursday. "We're going to have about 500 to 600-ish troops there, at the end of the day," he said.

Asked whether this figure includes about 200 troops deployed at Al-Tanf base near the southeastern borders with Jordan and Iraq, Esper said that he referred to northeastern Syria, where Trump has instructed the Pentagon to secure oil fields.

According to him, numbers could fluctuate, especially if European allies strengthen their presence in the country.

"Things change. Events on the ground change. We could have, for example, partners and allies from Europe joining us," he said. "If they join us on the ground it may allow us the ability to redeploy further U.S. forces out there."