BEIRUT (Reuters) - U.S.-backed Syrian militias have captured a town south of the city of Raqqa where Islamic State ran a major military base and training camp, a spokesman for the militias said on Tuesday.

An alliance of Kurdish and Arab militias fighting under the banner of the Syrian Democratic Forces (SDF) is trying to oust Islamic State from its headquarters in Raqqa.

SDF media official Mustafa Bali said the militias had seized the town of al-Ukeirshi, some 15 km (10 miles) downstream from Raqqa on the Euphrates river.

The SDF pushed into Raqqa last month after a long offensive, backed up by air strikes and special forces from the U.S.-led coalition.

A series of recent advances along the southern bank of the Euphrates have allowed the SDF to completely besiege the militants inside Raqqa and to press on south of the city.

The SDF alliance, spearheaded by the Kurdish YPG militia, has also been waging fierce battles inside the Old City of Raqqa since last week, after U.S.-led coalition jets breached its historic walls.

In al-Ukeirshi, Islamic State killed scores of its own forces execution-style in 2015 for desertion or on accusations of treachery, said the Britain-based Syrian Observatory for Human Rights.

Islamic State named the training camp it established in the town after Osama bin Laden, the founder of the al-Qaeda militant group killed by U.S. forces in 2011.