The US decision to not immediately withdraw from Syria is an indication of political differences in the United States, analysts said.

US President Donald Trump announced on Dec 19 that he would pull back 2,000 US troops from Syria "now", causing concern among US allies and many US officials.

John Bolton, the national security adviser, said on Sunday that the US would pull out only after the Islamic States group is sure to be defeated and Kurdish militias are safeguarded.

"We're going to be discussing the president's decision to withdraw, but to do so from northeast Syria in a way that makes sure that ISIS is defeated and is not able to revive itself and become a threat again," Bolton said when meeting Israeli Prime Minister Benjamin Netanyahu in Jerusalem.

Bolton was quoted by The Associated Press as saying that another condition for the US to pull out is for the US administration to reach an agreement with Turkey to protect the Kurds who have fought alongside US troops against the extremists.

Turkey and the US agreed last month to complete an agreement for the withdrawal of the Syrian Kurdish People's Protection Units militia (known by the acronym YPG) from the northeast Syrian town of Manbij before the full US withdrawal from Syria.

Bolton said the US wants its Kurdish allies in Syria 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," Bolton said.

He said that in meetings with Turkish officials that were scheduled on Jan 7, he would seek "to find out what their objectives and capabilities are and that remains uncertain".

In a Dec 19 video posted on social media, Trump said of US troops: "They're all coming back, and they're coming back now."

Trump also promised a "strong, deliberate and orderly withdrawal" of forces from Syria when he visited Al Asad Air Base in Iraq, where some US troops are stationed.

Yuan Zheng, a researcher from the Institute of American Studies at the Chinese Academy of Social Sciences, said Bolton's speech reflected major differences among decision-makers.

"People in the military and Congress are not satisfied with Trump's decision to pull out from Syria as it's not in the interests of the US," he said. "It's hard to say how they persuaded or pressured the president, but the outcome is that Washington will not leave Syria as soon as Trump has claimed."

He said the timetable is yet to be decided, or at least is not likely to be within a short period of time, until the US decides it has achieved its goal: the power to affect the future of Syria.

The changing US decision, Yuan said, could make the situation in Syria more complicated and disrupt the process of reconciliation between pro and anti-government forces.

"Opposition groups said they wished to propose a settlement plan after the withdrawal decision was announced, which could have promoted the reconciliation process," he said. "But now it's hard to say if they still want to compromise, especially when their backup-US troops-will stay for quite a while."