BEIRUT: U.S.-backed fighters have captured 80 percent of Syria's Tabqa from ISIS, an activist group said on Monday, a week after they first entered the town.

The Syrian Democratic Forces (SDF) broke into Tabqa from the south last week and have steadily advanced north, cornering ISIS in three contiguous neighborhoods on the bank of the Euphrates River.

The strategic town of Tabqa sits on a supply route about 55 kilometers (35 miles) west of Raqqa city, the de facto capital of ISIS territory in Syria.

At dawn, ISIS fighters withdrew from the western-most district towards the other two quarters, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

"The SDF now controls more than 80 percent of Tabqa," Abdel Rahman said.

"In the whole town, ISIS only holds those two neighborhoods, known locally as the first and second quarters," he told AFP.

Clashes and bombing raids by the U.S.-led coalition were rocking the town on Monday morning, the Observatory reported.

The assault on Tabqa began in late March when SDF forces and their U.S.-led coalition allies were airlifted behind ISIS lines.

The SDF -- composed of Arab and Kurdish fighters -- then surrounded Tabqa in early April before pushing into the town on April 24.

The assault on Raqqa, dubbed "Wrath of the Euphrates", was launched in November and has seen SDF fighters capture large swathes of countryside around the city.

More than 320,000 people have been killed in Syria since the country's war began with anti-government protests in March 2011.