President Donald Trump's national security adviser, John Bolton, says the U.S. military withdrawal from northeastern Syria is conditioned on defeating the remnants of the Islamic State group, and Turkey assuring the safety of Kurdish fighters allied with the United States.

Bolton, who is in Israel to reassure the U.S. ally of the Trump-ordered withdrawal, says there is no timetable for the pull-out of American forces in northeastern Syria, but insists it's not an unlimited commitment.

Bolton said on Sunday that "the timetable flows from the policy decisions that we need to implement."

His comments mark the first public confirmation the drawdown has been slowed, as Trump faced widespread criticism from allies and the resignation of Defense Secretary Jim Mattis for a policy that was to have been conducted within weeks.