Trump says US withdrawal from Syria will be 'prudent'

US President Donald Trump has gone back on a previous promise to swiftly pull-out from Syria saying the withdrawal would be done in a "prudent" manner.

Trump said the fight against the Islamic State group will continue until they have been defeated.

"We will be leaving at a proper pace while at the same time continuing to fight ISIS and doing all else that is prudent and necessary!" Trump tweeted, using an acronym for the Islamic State.

This follows the announcement in late December that American troops would be withdrawn from Syria immediately.

"We've won against ISIS," he said at the time. "We've beaten them and we've beaten them badly. We've taken back the land. And now it's time for our troops to come back home."

Trump's security adviser John Bolton told Israeli Prime Minister Binyamin Netanyahu on Sunday that withdrawal would not happen before "ISIS is defeated and not able to revive itself".

The speedy withdrawal of American soldiers have sparked fears that a IS resurgence in Syria could be in the making, or that Turkish forces could launch an offensive against Kurdish fighters. 

The decision has led to a rebellion in Trump's Republican Party with the resignation of Defence Secretary James Mattis.

Trump complained on Monday that media coverage had skewed his original words, saying that his latest position on Syria was "no different from my original statements".

Bolton also implied that he told Turkey the US would not tolerate Kurdish fighters from the SDF being killed.

"That's what the president said, the ones that fought with us," he said.

He also urged Washington's Kurdish allies not to seek assistance from Russia or the Syrian regime.

Bolton also assured that some American troops would stay in the US manned base of al-Tanf in southern Syria, to counter the spread of Iranian militias in the area.

In another sign that the US might entrench its positions in Syria, the observatory also said that American forces were establishing a military base near Tal Abyad, which lies on the Turkish border.

"The international coalition renewed its contract to support the Syria Democratic Forces for two years... the contract is based on establishing a full programme for the coalition and a way for applying it, and then renewing it every two years," the report stated.

The monitor also announced that other SDF and American bases being established in eastern and northern Syria.