Washington: As many as 600 US troops will remain in northeast Syria to continue counterterrorism operations against the Islamic State, General Mark Milley, chairman of the Joint Chiefs of Staff, said Sunday.

Some to remain: US forces patrol oil fields in eastern Syria. Credit:AP

"There will be less than 1000 for sure," Milley said, referring to the number present when President Donald Trump ordered their complete withdrawal last month.

Trump later was persuaded by national security advisers and congressional supporters, such as Senator Lindsey Graham to retain an unspecified number of troops whose mission, the president said, was to "secure the oil" from a takeover by the Syrian government or militants.

Milley, speaking on the ABC News program This Week, said the number of troops that would remain was "probably in the 500-ish frame. Maybe 600." He did not mention Syrian oil, but said that "there are still ISIS fighters in the region and unless pressure is maintained ... then there's a very real possibility that conditions could be set for reemergence of ISIS."