John Bolton said on Saturday the US military withdrawal from Syria will only happen once the remnants of the so-called Islamic State group are defeated.
He added that Turkey needed to assure the safety of Kurdish fighters allied with the United States in the area.
Mr Bolton, who has been visiting Israel to reassure the US ally of the Trump-ordered withdrawal, said there is no timetable for the pullout of American forces in north-eastern Syria, but insists it is not an unlimited commitment.
He also said that "the timetable flows from the policy decisions that we need to implement".
Mr Bolton's comments marked the first public confirmation that the withdrawal has been slowed.