US forces are still operating in the northern Syrian city of Manbij/AFP
The withdrawal of US troops from Syria depends on certain conditions, US 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, reports BBC.
When he first announced the move in the middle of December, he said: "They're all coming back and they're coming back now."
After the president made his announcement on 19 December, US officials said American forces had been given 30 days to leave Syria.
In his announcement, Mr Trump had also declared that IS had been "defeated".
However, speaking on Sunday before leaving for his Camp David retreat, he told reporters: "We're going to be removing our troops. I never said we were doing it that quickly.
"We're pulling out of Syria... and we won't be finally pulled out until Isis [IS] is gone."