BEIRUT (Reuters) - U.S.-backed forces fighting Islamic State in Syria launched a new phase of their offensive on Thursday, a statement said, but they have not yet begun to attack the militant group’s stronghold of Raqqa city in an apparent delay in the operation.

FILE PHOTO: A view shows part of the northern part of the Tabqa Dam on the Euphrates River, Syria March 28, 2017. REUTERS/Rodi Said/File Photo

The multi-phased campaign by the Syrian Democratic Forces (SDF), an alliance made up of Syrian Kurdish and Arab fighting groups, was launched in November and aims ultimately to drive the jihadists from Raqqa, their de facto Syrian capital.

Officials in the Kurdish YPG militia, a powerful component of the SDF, said last month that assaults on Raqqa city itself would start in early or mid-April.

But the fourth phase of the campaign aims to clear Islamic State pockets from the countryside north of the city, the SDF statement said. It did not say when the assault on Raqqa itself would begin.

“We aim to liberate dozens of villages in the Wadi Jallab area and the northern countryside ... and clear the last obstacles in front of us to pave the way for the operation to liberate Raqqa city,” it said.

The SDF have closed in on Raqqa from the north, east and west.

They have surrounded the Islamic State-held Tabqa area and its adjacent dam, some 40 km (25 miles) to the west of Raqqa, which is the focus of heavy fighting and where Islamic State has launched a number of counter-attacks.

Officials have said that the assault on Raqqa could be launched even before Tabqa itself is seized, but the battles there have kept the SDF busy.

“The dam issue has taken some time,” Jihan Sheikh Ahmed, the SDF spokeswoman for the Raqqa campaign, told Reuters.

Forces were taking care to avoid damage to the dam, she added, which the U.N. had warned in February could cause catastrophic flooding because of the fighting.

Ahmed said the SDF were making simultaneous advances on the front lines around Raqqa, Tabqa and Deir al-Zor, another IS stronghold further southeast.