An alliance of Kurdish and Arab rebels advanced Thursday against the Islamic State jihadist group in Syria's north in a new offensive backed by US-led air strikes, its spokesman said.

Talal Sello, spokesman for the Syrian Democratic Forces, told AFP the SDF offensive was launched late Wednesday in Aleppo province.

"The battle now is aimed at the liberation of areas south of the town of Sarrin... to the Tishreen Dam" on the Euphrates River, Sello said.

Kurdish forces pushed IS out of Sarrin in July.

SDF fighters, which predominantly hail from the Kurdish People's Protection Units (YPG), had advanced to within 12 kilometres (less than eight miles) from the dam by Thursday, according to Sello.

The dam, been held by IS since 2014, helps generate electricity for large parts of Aleppo province, according to the Syrian Observatory for Human Rights.

IS also controls the Euphrates' western bank -- from the northernmost point at Jarablus, which lies on the Syrian-Turkish border, south and further east all the way to Raqa.

Manbij and Al-Bab, IS's strongholds in Aleppo province, lie west of the river as well.

Syrian forces, backed by Russian air strikes, have made gains against IS south of those towns.

"Fierce clashes are continuing along the eastern banks of the Euphrates," Observatory head Rami Abdel Rahman said Thursday.

He confirmed that the SDF had advanced along the banks, but said most of the territory captured consisted of small villages.

Sello said the SDF was receiving backing from the US-led coalition conducting an air war against IS in both Syria and Iraq.

A coalition statement Thursday said strikes conducted the previous day hit near Manbij and destroyed an IS fighting position and a logistics facility.

Formed in October, the SDF has scored a series of successes against IS in northeastern Syria and appears to be extending its operations further west.