AMMAN (Reuters) - U.S-led coalition planes bombed an Islamic State-controlled town near the Euphrates Dam in northern Syria a day after the launch of a new phase of a campaign to capture the militants' defacto capital of Raqqa, activists and the militants said on Sunday.

Activists confirmed reports released by the militants' news agency Amaq which said four raids in the last twenty four hours hit the town of Tabqa west of Raqqa, located near Syria's largest dam, at the southern end of Lake Assad on the Euphrates.

A video released by Amaq showed extensive damage to a commercial center in the town but did mention any casualties.

Islamic State fighters are battling hard to keep the swathes of territory they have captured in Syria, mostly in the center and east, as they lose ground in Iraq.

On Friday, air strikes in Raqqa hit at least four bridges over the Euphrates River, part of a weeks-long campaign of air strikes in the province and near the dam, activists said.

The Euphrates Dam is one of the major sources of electicity in the country and crucial for irrigation of farms in the fertile region around Raqqa.

On the ground, the Syrian Democratic Forces (SDF), an alliance of U.S.-backed militias, started a new phase of its campaign against Raqqa on Saturday, aiming to surround it and sever the road to militant strongholds in Deir al-Zor province.

The SDF, which includes the powerful Kurdish YPG militia, launched the campaign to capture Raqqa in November.

The alliance of militias quickly gained territory to the north of the city at the beginning of the campaign and has been targeting areas to the west of it. It is the main U.S. partner in the fight against Islamic State in Syria.

(Reporting by Suleiman Al-Khalidi; Editing by Louise Ireland)