The United States has no plan for Syria as it proceeds with President Donald Trump's order to pull troops out of the country, Washington's former anti-ISIL envoy, who quit in protest against the withdrawal, said.

Brett McGurk, who was the envoy to the US-led global coalition against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, said on Sunday that "there is no plan for what's coming next" and this increases the risk to the US forces.

190116115319189

He spoke in an interview with CBS's Face the Nation, after a suicide bomber on Wednesday killed four US military personnel and 15 others in the northern Syrian town of Manbij.

It was the deadliest attack on the US troops since their deployment in Syria in 2014 to assist the local forces fighting ISIL.

The bombing came after Trump's announcement last month that he was ordering a full withdrawal of the 2,000 US troops from Syria, shocking allies and prompting the resignations of Defence Secretary Jim Mattis as well as McGurk.

'Withdrawal began'

Senior US officials have since given contradictory statements about the US intentions. On January 10, the Pentagon said the withdrawal process had begun. It started with the removal of equipment, not troops, according to the Pentagon. It is uncertain how long a full withdrawal will take.

"The president has made that clear - we are leaving. And that means our force should be really with one mission: to get out and get out safely," McGurk told "Face the Nation".

But he added: "Right now we do not have a plan. It increases the vulnerability of our force... It is increasing the risk to our people on the ground in Syria and will open up space for ISIS."

190121011835123

Most importantly, said McGurk, the US cannot expect "a partner" such as NATO-ally Turkey to take the place of the US.

"That is not realistic. And if our forces are under order to withdraw, as at the same time they are trying to find some formula for another coalition partner to come in, that is not workable. That is not a viable plan."

Trump announced the US withdrawal because, he said, ISIL had been defeated - something McGurk and other experts dispute.

McGurk has previously warned that the US pullout would shore up Syria's President Bashar al-Assad and lessen the US's leverage with Russia and Iran.