BEIRUT (Reuters) - A U.S.-backed alliance of militias has made unexpectedly rapid gains against Islamic State near its Syrian headquarters in Raqqa, advancing to within a few kilometres of a major dam, its spokesman said.

The Syrian Observatory for Human Rights, a UK-based organisation that reports on the conflict, said one of Islamic State’s top commanders in Syria was probably killed in fighting near the dam on the Euphrates River some 50 km (30 miles) west of Raqqa city, Islamic State’s main base of operations in Syria.

The Syrian Democratic Forces (SDF) alliance, including the powerful Kurdish YPG militia, launched the second phase of a campaign against Islamic State 10 days ago. The ultimate aim is to capture Raqqa city.

The latest phase is targeting IS-held areas to the west of the city, including the dam near Tabqa. The first phase, launched in early November, targeted IS positions to the north.

“1,300 square km (captured from Islamic State) in 10 days - we were expecting it to take longer, but the Daesh (Islamic State) defences disintegrated,” Silo said in a phone interview. Fifteen SDF fighters had been killed in the latest advances, he said.

IS was deploying suicide car bombs in its efforts to fend off the assault, Silo said, adding that these were being destroyed before reaching their targets thanks to anti-armour weapons recently supplied by the U.S.-led coalition.

U.S. special forces were working “side by side with our forces”, he said.

The Observatory identified the Islamic State commander killed in the latest fighting as Abu Jandal al-Kuwaiti. Observatory Director Rami Abdulrahman said he was killed as IS fighters sought to recapture the village of Jabar, which the SDF captured on Monday. The counter-attack failed.