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.
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."
Last month's announcement 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 Mr 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.
What did John Bolton say?
"We don't think the Turks ought to undertake military action that is not fully co-ordinated with and agreed to by the United States at a minimum so they don't endanger our troops, but also so that they meet the president's requirement that the Syrian opposition forces that have fought with us are not endangered," Mr Bolton said in Israel ahead of talks with Prime Minister Benjamin Netanyahu.
He told reporters there was no timetable for a US withdrawal from Syria but that there was not an unlimited commitment.
Mr Bolton also said President Trump wanted to ensure that IS was "destroyed".
Mr Netanyahu said he would discuss Iran's ambitions in Syria when he met Mr Bolton on Sunday evening.
On Tuesday, US Secretary of State Mike Pompeo will begin a week-long tour of the Middle East designed to reassure allies in the region.
The US partnership with the Kurdish-led Syria Democratic Forces (SDF) is credited with playing a major role in the demise of IS.
But the main fighting force in the SDF is the Kurdish YPG militia, which Turkey regards as a terrorist group.
On Sunday, a spokesman for Turkish President Recep Tayyip Erdogan said it was "irrational" to suggest Turkey targeted Kurds.
Ibrahim Kalin said Turkey's focus was on the YPG and the Kurdistan Workers' Party (PKK) - which Ankara also regards as a terror group.
What is the US presence in Syria?
Some 2,000 US military personnel are reported to be in Syria although the real numbers could be higher.
American ground troops first became involved in Syria in autumn 2015 when then-President Barack Obama sent in a small number of special forces to train and advise local Kurdish fighters who were fighting IS.
The US did this reluctantly after several attempts at arming anti-IS groups had descended into chaos.
Over the intervening years the number of US troops in Syria has increased, and a network of bases and airfields has been established in an arc across the north-eastern part of the country.