US-backed militias claim big advance against IS in Syria's Tabqa

US-backed militias said on Sunday they had made a big advance in Tabqa, a strategically vital town controlling Syria's largest dam, in their campaign to drive Islamic State (IS) from its stronghold of Raqqa, 40km downstream.

The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias, will wait to assault Raqqa until after it seizes Tabqa, its military officials have previously said, but it had made only slow progress since besieging the town early this month.

In a statement it circulated on social messaging sites, the SDF said it had captured six more districts of Tabqa and distributed a map showing that IS now controlled only the northern part of the town, next to the dam.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said the SDF had gained almost complete control over Tabqa.

The SDF last Tuesday was hit by air strikes from Turkey, which fears the advance of the SDF's main component, the YPG militia, in Syria. Twenty eight people died in the Turkish strike, according to the Observatory.

The US condemned the strikes, which came without warnings to Ankara's Nato partners.

Tabqa was isolated from Raqqa in late March after the US helped the SDF carry out an airborne landing on the southern bank of the Euphrates, allowing it to capture the areas around the town, including an important airbase, and cut the road.

IS still holds several Tabqa districts along the southern bank of Lake Assad and the southern section of the Euphrates dam, including its operations facilities and a hydro-electric power plant.

Raqqa now lies in an IS enclave on the northern bank of the Euphrates that measures about 50km at its widest point on an east-west axis and 20km on its longest north-south axis.

Islamic State's only means of crossing to its main territory on the south bank of the Euphrates is by boat after aerial bombing put the region's bridges out of service.

The militant group still controls large swathes of Syria's Euphrates basin and its vast eastern deserts near the border with Iraq, but it has lost large tracts of its territory over the past year and many of its leaders have been killed.

The SDF published photos on Sunday it said showed items retrieved from newly captured parts of the town, including at least a dozen guns, as well as missiles, ammunition, and an IS flag.

Tabqa is home to about 85,000 people, including IS militants from other areas.

IS has put up fierce resistance, including using weaponised drones, a tactic the group perfected in neighbouring Iraq.

The group is also fighting street-to-street and using suicide attackers and car bombs to slow the SDF's advance, according to the Observatory.

The assault on Raqqa, dubbed "Wrath of the Euphrates," was launched in November and has seen SDF fighters capture large swathes of countryside around the city.