A U.S.-backed alliance of Syrian Kurds and Arab rebel groups advanced toward a dam in northern Syria held by Islamic State fighters on Thursday, in an assault aimed at capturing the militants' stronghold of Raqqa, rebels and a monitor said.

They said the push by fighters of the Democratic Forces of Syria, which includes the powerful Kurdish YPG militia and an Arab tribal alliance, had brought them within 20 km of the Tishreen Dam, one of three major dams on the Euphrates River.

Further south of the dam that still supplies power to large areas in northern Syria is the even larger Baath Dam. Its power plant, under militant control, feeds much of the territory under Islamic State and is 22 km (15 miles) upstream from Raqqa.

Colonel Talal Selo, the spokesman of the alliance that has received support under a new U.S. strategy aimed at fighting Islamic State in Syria, told Reuters troops advancing from the northeastern Kurdish corner of Syria were able since Tuesday to seize several villages.

They include Sahareej, Obeidat and al Manseeh.

Across the border in Iraq, US-led coalition warplanes pounded positions held by Islamic State in the western city of Ramadi, according to the Iraqi military. Iraqi forces were preparing for a last push they hope will drive the militants from the city that was lost in May in a major blow to Baghdad.

SYRIA STRATEGY SHIFT

Washington's strategy in Syria has shifted this year from trying to train thousands of fighters outside the country to supplying groups headed by U.S.-vetted commanders.

U.S. officials said delivery of weapons since the alliance was set up would help the fighters push further south into Islamic State-held territory.

The U.S. military estimates the Democratic Forces of Syria has captured around 1,000 square kilometers of terrain in the past six weeks or so, bolstered by coalition air strikes.

Selo said the new focus on the area near the dam and south along the banks of river would help it isolate the militants' strongholds in northern Aleppo from their territories east of the Euphrates river, where Raqqa city, effectively their capital, is located.

"This will isolate Daesh and allow us to push further toward Raqqa, our ultimate goal to defeat these terrorists," he said, using a widely used derogatory Arabic term for the militants.

Since the U.S backed alliance was formed last October, its fighters have opened several major offensives against Islamic State, including one near the militant-held town of Haul near the border with Iraq.

Selo said their fighters were also fortifying positions near the militant run al-Shadadi town, a major logistics hub for the group, located on a strategic network of highways and whose capture would isolate Raqqa.

The British-based Syrian Observatory for Human Rights said on Thursday jet fighters believed to belong to the U.S.-led coalition had raided militant hideouts in the Shadadi area.

Separately, Islamic State militants took over a government controlled district in the eastern city of Deir Zor further south-east after several suicide car bombings in an advance that threatened to bring much of the city under their control.

The provincial capital on the banks of the Euphrates, 430 km (270 miles) northeast of Damascus, is focus of a vast, arid oil-producing region bordering Iraq.

Half of it fell to rebels in 2012 but Syrian forces have held out in several districts in the west of the Sunni Muslim city and in the airport to the east.