AMMAN/BEIRUT: U.S.-backed Syrian forces repelled a major counterattack by Daesh (ISIS) militants holding out at the country’s largest dam and in the nearby town of Tabqa, the group and activists said Sunday.

The dam is a key strategic target in the military campaign to isolate and capture the Syrian city of Raqqa, 40 kilometers to the east and Daesh’s biggest urban stronghold.

The U.S.-backed group said militants attacked their positions northeast of Tabqa and at an air base to the south of the town where dozens of their fighters were killed; but the coalition of Kurdish and Arab militias was making slower advances in a village east of the town.

Jihan Sheikh Ahmad, a spokeswoman for the Syrian Democratic Forces, spearheaded by the powerful Kurdish YPG militia, said the militants were stepping up their resistance as SDF forces got closer to encircling the town and the dam.

“Our forces are advancing ... [Daesh] are facing large difficulties and so they are starting counterattacks,” said the official, whose forces have U.S. special forces with them.

The SDF began an assault to capture the dam and the nearby town almost two weeks ago after the coalition landed some of its fighters on the southern side of the Euphrates near Tabqa, leading to its capture of an air base.

The SDF has been waging a multiphased offensive since November to isolate Raqqa, with backing from the U.S.-led coalition taking territory to the north and east of the province.

Hundreds of families with their cattle, property, motorbikes and vans continued Sunday to flee from villages under Daesh control. The U.S.-backed forces say at least 7,000 people have taken shelter in their areas since the campaign to capture Tabqa began. Many are also fleeing airstrikes on civilian areas in Raqqa province that have left dozens dead according to activists and the Britain-based Syrian Observatory for Human Rights. The SDF denies civilians are targeted.

Daesh and the Syrian government have both said the hydroelectric dam is vulnerable to collapse after strikes by the U.S.-led coalition. Syrian officials say that would lead to catastrophic flooding in the cities and towns in the Euphrates valley downstream. The SDF and the coalition have denied the dam is in danger.

Residents, however, say an SDF attempt last week to relieve the dam’s water levels by opening a canal from the Balikh River that flows into the Euphrates had flooded cultivated agricultural land in several villages.

Elsewhere, jets believed to be Russian hit an outpost run by moderate rebel forces in northwestern Syria near a major border crossing with Turkey, killing at least one fighter and wounding several people, two rebel sources said Sunday.

They said several raids overnight hit Babeska, a village in Idlib province that has become a haven for several moderate Free Syrian Army groups, mainly the Army of Islam, a major insurgent group that controls the last major rebel stronghold on the doorstep of the Syrian capital.The village also houses hundreds of families and fighters from the Damascus suburb of Daraya which was evacuated by rebels and surrendered to government control last year. War jets also believed to be Russian struck Urum al-Kubra town in rebel-held western Aleppo countryside where five civilians were killed, in an area that witnessed fighting between rebels and the Syrian army, rebels said.

In central Syria, hundreds of opposition fighters and their families left the city of Homs Saturday after being evacuated from the last rebel-held neighborhood of Waer. State news agency SANA said 1,860 people, including 531 fighters, left Waer toward the country’s north in the third evacuation from the district in two weeks. The evacuation deal was brokered by Russia, and Russian troops were seen in the city observing the evacuation.

Unlike the previous two evacuations, in which the fighters and their families headed to the town of Jarablus on the border with Turkey, Saturday’s evacuees headed toward the rebel-held province of Idlib.

In other developments, $262 million has been pledged in humanitarian aid for Syria following a meeting in Doha involving the U.N. and 25 non-governmental organizations from across the region, including Lebanon, Jordan and Turkey. Ahmed bin Mohammad al-Muraikhi, the U.N. secretary-general’s humanitarian envoy, told the meeting Syria faces a huge humanitarian crisis that requires up to $8 billion in aid for the current year.