A U.S.-backed Arab-Kurdish alliance advanced Wednesday to several kilometers from the largest prison held by the Islamic State jihadist group in Syria's northern province of Raqa, a monitor said.

Members of the Syrian Democratic Forces (SDF) were eight kilometers (five miles) from the jail near the country's largest dam at Tabqa on the Euphrates River, the Syrian Observatory for Human Rights said.

On December 10, the SDF announced "phase two" of its campaign against IS' de facto Syrian capital of Raqa some 50 kilometers east of Tabqa in the same province.

The alliance has since captured dozens of villages and hamlets, the Observatory says, after taking 700 square kilometers (270 square miles) from the jihadists in a first phase of the assault from November 5.

Western hostages of the jihadist group are believed to have been held at the jail near the town of Tabqa, where top IS leaders live, the Britain-based Observatory said.

Kurdish officials told AFP on Wednesday that many of the jihadists had fled before the SDF advance.

IS fighters "have no more heavy weapons -- they are weakened and flee without resisting," said Moslem Sheikh Nouh, a commander with the alliance.

"IS is not resisting, absolutely not," she said, adding that the jihadists "take to their heels in flight."

The SDF is backed by air strikes from a U.S.-led coalition as well as by some U.S. forces on the ground.

Washington said this month it was sending 200 additional troops to join the 300 it has already deployed to support the Raqa campaign.

Despite recent setbacks, IS still controls areas of Syria and neighboring Iraq, where it has faced an offensive since October on its main bastion of Mosul.

This month, IS seized the ancient city of Palmyra in central Syria for the second time, just eights months after President Bashar Assad's army backed by Russia drove the jihadists out.

IS overran large parts of Syria and Iraq in 2014, proclaiming Raqa the capital of its "caliphate", after Syria's war started in 2011 with the brutal repression of anti-government protests. </s> BEIRUT, June 4 (Reuters) - U.S.-backed Syrian militias said they seized a major dam on the Euphrates river from Islamic State on Sunday, their latest gain as they push towards Raqqa city.

The Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, said they captured the Baath Dam in the morning, renaming it Freedom Dam. The hydroelectric dam lies some 22 km upstream of Raqqa, Islamic State's base of operations in Syria.

The spokesman for the Kurdish YPG militia, the SDF's key component, said fighters were combing nearby villages for mines and shoring up their defensive lines. "The dam has been completely liberated," Nouri Mahmoud said.

The advance means the SDF now hold all three major dams along the Euphrates, after gaining control of Syria's largest dam last month.

With air strikes and special forces from the U.S.-led coalition, the SDF have been encircling Raqqa to take the city, which Islamic State has used as a hub to plan attacks abroad.

The alliance advanced in recent months to within several kilometres (miles) of the centre of Raqqa, facing fierce resistance from Islamic State. Fighting since late last year has displaced tens of thousands of people according to United Nations sources, with many flooding camps in the area.

The operation to storm the city will start in the next "few days", Mahmoud had said on Saturday. The Raqqa assault will pile more pressure on the jihadist group's self-declared "caliphate" as it faces defeat in the Iraqi city of Mosul and retreats across much of Syria.

Islamic State still controls swathes of Syria's eastern deserts bordering Iraq and most of Deir al-Zor province, which would be its last major foothold in Syria they lose Raqqa. (Reporting by Ellen Francis; Editing by Catherine Evans)
U.S.-backed Syrian Democratic Forces advance to eight kilometers (five miles) from the largest prison held by ISIL near the Tabqa Dam.