BEIRUT (Reuters) – U.S.-backed forces fighting Islamic State in Syria took several neighborhoods from the militant group in the town of Tabqa on Friday, they said in a statement, part of a campaign to oust Islamic State from its stronghold in Raqqa city.

The multi-phased campaign against the jihadists by the Syrian Democratic Forces (SDF), an alliance made up of Syrian Kurdish and Arab fighting groups, was launched in November but has slowed in recent weeks.

Pushing down from the north, the SDF is trying to take the Islamic State-held Tabqa area and its adjacent Euphrates dam, the largest in Syria, some 40 km (25 miles) upstream of Raqqa.

SDF forces surround Tabqa town, having cut it off in late March from a swathe of Islamic State territory which runs across Syria into Iraq.

On Friday the SDF said it had pushed up into the town and taken the southern neighborhoods of Nababila and Zahra, having taken Wahab neighborhood to their south on Thursday.

In recent weeks the SDF has also squeezed Islamic State’s pocket of territory around Raqqa, which the jihadist group has used as a base to plot attacks and manage much of its self-declared caliphate since seizing the city in 2014.

The Kurdish YPG militia is the strongest unit of the SDF and is taking part in the assault on Tabqa and Raqqa, but it is seen by Turkey as an extension of the Kurdistan Workers Party (PKK), which has fought a three-decade insurgency against Ankara.

On Tuesday the Turkish military conducted air strikes and cross-border shelling against YPG targets in Syria in what it said was retaliation for mortar attacks, prompting the U.S. State Department to voice concern. Sporadic clashes have continued along the border in recent days.

(Reporting by Lisa Barrington; Editing by Gareth Jones)