The withdrawal of American troops from Syria depends on certain conditions, United States National Security Adviser John Bolton says, in a further indication that the process is being slowed down.
On a trip to Israel and Turkey, he said he would seek Turkish assurances that Kurds in northern Syria would be safe.
The US also wants to ensure that the remnants of the Islamic State (IS) group are defeated, he added.
President Donald Trump has faced strong criticism over the planned US pullout.
When he first announced the move in the middle of December, he said: “They’re all coming back and they’re coming back now.”
US officials said American forces had been given 30 days to leave Syria.
Mr Trump also said at the time that IS had been “defeated”.
The development shocked allies and US defence officials alike, with Defence Secretary Jim Mattis and senior aide Brett McGurk resigning soon after. On Saturday, Department of Defence chief of staff Kevin Sweeney became the third senior Pentagon official to announce his resignation since President Trump’s announcement.
Meanwhile, America’s Kurdish allies in north-east Syria were left feeling exposed as Turkey, which regards them as terrorists, appeared poised to move against them.
But Trump seemed to row back last week when he said troops were being pulled out “slowly” and that they would be fighting remaining IS militants at the same time.