The battle to capture the Islamic State group's Syrian bastion Raqa will begin ‘in days’, a militia spokeswoman said Saturday after fresh advances by a Kurdish-Arab alliance battling the jihadists.

The US-backed Syrian Democratic Forces (SDF) began an operation to capture Raqa last November and has gradually closed in on the city, seizing a town and dam to the west overnight and on Saturday.

Their fighters are now within a few kilometres (miles) of the jihadist stronghold to the north, east and west, and are expected to launch their final assault soon.

‘We will begin in a few days,’ said Jihan Sheikh Ahmed, spokeswoman for the SDF's ‘Wrath of the Euphrates’ operation to capture Raqa.

SDF fighters have already sealed off the approaches to Raqa from the north and east, and are close to doing the same with the western approach after new advances.

The Britain-based Syrian Observatory for Human Rights monitor said the SDF captured the town of Mansura late Friday and the adjacent Baath Dam on the Euphrates River, around 20 kilometres (10 miles) west of Raqa, on Saturday.

‘This advance will allow the SDF to expand its control on the southern banks of the Euphrates River and stabilise the western front of Raqa before launching the final battle to expel IS from the city,’ Observatory director Rami Abdel Rahman said.

‘We're nearing the major battle,’ he added.

- Coalition air cover -

Abdel Rahman said combing operations were ongoing in Mansura and at the dam to ‘dismantle mines and search for remaining jihadists’.

SDF spokesman Talal Sello told AFP the advances on the western front were part of the final stage of operations before the assault on Raqa begins.

He said the SDF had received ‘weapons and advanced equipment from the international coalition... as part of preparations for the launch of the battle for Raqa, which is close’.

An AFP correspondent in Ain Issa, a key staging ground for SDF operations, on Saturday saw a convoy of armoured vehicles driven by foreign advisers from the US-led coalition against the jihadist group.

Trucks carrying bulldozers and other machinery were also in the convoy.

Sello said the SDF would launch the attack from the north, west and east of Raqa.

‘The SDF has already completed the siege from the northern and eastern sides and is working to complete the siege from the west,’ he added.

The SDF is fighting with broad support from the US-led coalition, which has provided air cover, assistance from special forces on the ground and weaponry.

All major routes into the city have now been severed.

IS fighters are still able to move out of the city to the south, however, crossing the Euphrates by boat before continuing on through the desert.

The SDF is not expected to try to seal the southern route before starting its final assault.

‘The SDF doesn't need to isolate Raqa from the south, because the international coalition's planes can target any jihadists as they cross the river,’ Abdel Rahman said.

Related Story