Qamishli, Syria - The US forces that led the coalition against the Islamic State group will complete their withdrawal from Syria within a month, three sources told AFP on Monday, as troops began leaving a major base.

The withdrawal comes as Syria's government expanded its control to the country's northeast, previously controlled by US-allied Kurdish forces, and formally joined the coalition against IS.

It also comes as Syrian state media reported that four Syrian security personnel were killed in an IS attack in the northern city of Raqa, which was recently taken back into central government control from Kurdish forces.

American forces have already withdrawn from two more bases in the past two weeks, Al-Tanf in the southeast and Shadadi in the northeast.

Cats Cat owner finds her kitties lined up in a row for this hilarious reason in viral clip

"Within a month, they will have withdrawn from Syria, and there will no longer be any military presence in the bases," a Syrian government official said, with a Kurdish source confirming the timeline.

The officials who spoke to AFP for this story all requested anonymity because they are not authorized to speak to the media.

On Monday, the US began withdrawing from a major base in a northeastern region still under the control of Kurdish forces, which agreed last month to integrate their institutions with Damascus.

An AFP team saw a convoy of dozens of trucks loaded with armoured vehicles and prefabricated structures on a road linking the Qasrak base in Hasakeh province to the border with Iraq.

With Kurdish forces at the forefront, IS was territorially defeated in 2019 but retains sleeper cells, and on Saturday, the group urged its jihadists to fight Syrian authorities.

On Monday, Syria's official SANA news agency quoted a security source as saying that "four members of the internal security forces" were killed in an attack attributed to IS.

Syria's interior ministry said the "terrorist attack" had targeted a checkpoint and that an assailant was killed.

The US has about 1,000 troops still deployed in Syria.

It had intervened in the country in 2014 to fight IS, which had taken over swathes of Syria and Iraq in a lightning offensive.