White House national security adviser John Bolton added a new condition on Sunday to the slowing US withdrawal from Syria, saying Turkey must agree to protect the United States' Kurdish allies.

Key points: John Bolton's comments suggest the withdrawal will take longer than first thought

John Bolton's comments suggest the withdrawal will take longer than first thought Donald Trump has insisted it will still happen, but not "quickly"

Donald Trump has insisted it will still happen, but not "quickly" Turkey has rejected accusations it would harm the Kurds

President Donald Trump's abrupt decision to announce a US pull-out from Syria left many questions, chiefly whether Kurdish fighters operating in northern Syria would now be targeted by their long-time enemy Turkey.

The decision to withdraw, which also led to the resignation of US Defence Secretary Jim Mattis and other senior staff, has raised fears it will clear the way for a Turkish assault on the Kurdish fighters.

Turkey considers the Syrian Kurdish People's Protection Units, or YPG, a terrorist group linked to an insurgency within its own borders.

Mr Bolton, on a four-day trip to Israel and Turkey, said he would stress in talks with Turkish officials, including President Tayyip Erdogan, that the Kurds must be safeguarded.

"We don't think the Turks ought to undertake military action that's not fully coordinated 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 told reporters ahead of talks with Israeli officials.

Sorry, this video has expired Donald Trump announces US withdrawal from Syria

In Washington on Sunday, Mr Trump reiterated that the US would be pulling its troops out of Syria but suggested the move might not happen soon.

"I never said we are doing it that quickly. But we are decimating ISIS," Mr Trump told reporters.

Mr Bolton, who will travel to Turkey on Monday, said the US would talk to Turkey to find out what its objectives and capabilities were.

But he said Mr Trump's position was Turkey may not kill the Kurds and that the US withdrawal would not take place without an agreement on that.

Jim Jeffrey, the special representative for Syrian engagement and the newly named American special envoy for the anti-Islamic State coalition, is to travel to Syria this coming week in an effort to reassure the Kurdish fighters that they are not being abandoned, Mr Bolton said.

Turkey's presidential spokesman called allegations that his country planned to attack the US-allied Kurds in Syria "irrational" and said Turkey was fighting terrorism for national security.

In comments carried by the official Anadolu news agency, Ibrahim Kalin said the Kurdish fighters oppressed Syrian Kurds and pursued a separatist agenda under the guise of fighting IS.

"That a terror organisation cannot be allied with the US is self-evident," he said.

Mr Bolton said US troops would remain at the critical area of al-Tanf, in southern Syria, to counter growing Iranian activity in the region. He defended the legal basis for the deployment, saying it's justified by the President's constitutional authority.

The US is also seeking a "satisfactory disposition" for roughly 800 IS prisoners held by the US-backed Syrian opposition, Mr Bolton said, adding talks were ongoing with European and regional partners about the issue.

ABC/wires