Mark Milley
Mark MilleyReuters

General Mark Milley, chairman of the Joint Chiefs of Staff, said on Sunday that US troop levels in northern Syria will probably stabilize around 500.

"There will be less than 1,000, for sure. Probably in the 500ish frame, maybe six," Milley told ABC News in an interview.

The comments follow US President Donald Trump unexpected announcement last month that he would be withdrawing troops from northern Syria.

However, Defense Secretary Mark Esper later said that the US is mulling the possibility of maintaining a small ground presence near the oil fields in the area.

Trump's abrupt announcement that he had ordered a full troop withdrawal drew angry rebukes at home and abroad, with critics saying it could allow a resurgence of the Islamic State (ISIS) group while leaving US-allied Kurdish fighters in Syria vulnerable to a Turkish invasion.

In fact, Turkey launched a military operation against Kurdish fighters in northern Syria days after Trump’s withdrawal announcement.

The US and Turkey subsequently agreed on a ceasefire during which Kurdish fighters would withdraw from northern Syria.

Milley, who has commanded troops in Afghanistan and Iraq, told ABC on Sunday that it was important for US troops to remain in Syria so long as ISIS has a presence there.

"There are still ISIS fighters in the region. Unless pressure is maintained, unless attention is maintained on that group, there's a very real possibility there could be a re-emergence of ISIS," said Milley.

Asked about the killing of ISIS leader Abu Bakr al-Baghdadi by a US special forces unit late last month, Milley said it would have a "significant disruptive effect on the organization."

He stressed that the US has "a considerable amount of information on his successor," Abu Ibrahim al-Hashimi al-Quraishi.

"Where opportunities arise, we'll go after him," said Milley.