President Donald Trump's national security adviser is visiting Turkey for talks on the planned withdrawal of American forces from Syria.
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.
President Trump's announcement that the US would withdraw all its troops in Syria faced widespread criticism.
It led to the resignation of Defense Secretary General James Mattis, who said in his resignation letter "you (President Trump) have a right to have a Secretary of Defense whose views are better aligned with yours".
President Trump's plans raised fears over clearing the way for a Turkish assault on Kurdish fighters in Syria who fought alongside American troops against IS.
Turkey considers the Syrian Kurdish People's Protection Units, or YPG, a terrorist group linked to an insurgency within its own borders.
Mr Bolton said the US is insisting that its Kurdish allies in the fight against IS are protected from any planned Turkish offensive.
"We don't think the Turks ought to undertake military action that's not fully coordinated with and agreed to by the United States," Mr Bolton said.
"President Trump has stated that he would "not allow Turkey to kill the Kurds.
"That's what the president said, the ones that fought with us."
Mr Bolton said the US has asked its Kurdish allies to "stand fast now" and refrain from seeking protection from Russia or Syrian President Bashar al-Assad's government.
He added that US troops would remain at the critical area of al-Tanf, in southern Syria, to counter growing Iranian activity in the region.
According to reports, two British special forces soldiers were seriously injured by an IS missile in Syria on Saturday morning.
The UK has been targeting IS fighters since 2014 as part of Operation Shader, with fighter jets flying daily missions over Iraq and Syria.