BEIRUT (Reuters) - U.S.-backed forces captured a town west of Raqqa from Islamic State on Friday as they push toward the militants’ Syrian stronghold, a security source and a monitoring group said.

The United States said this week it started distributing weapons to Kurdish YPG militiamen who form a key part of the Syrian Democratic Forces (SDF) alliance which Washington is backing in its fight against the jihadists.

“It’s been around three days that the SDF has been attacking al-Mansoura. Today, from two sides, they entered, and it was liberated,” a security source close to the SDF told Reuters.

The British-based Syrian Observatory for Human Rights said al-Mansoura was the largest town in the western Raqqa countryside. There were still some clashes taking place around pockets of IS resistance in the town, it said.

The SDF, which includes the YPG and Syrian Arab fighting groups, said last month it expected to storm Raqqa in early summer as part of a multi-phased offensive.

The alliance has advanced in recent months to within several kilometers (miles) of the center of Raqqa, although their gains have taken longer than predicted with fierce resistance from IS.

Fighting since late last year has displaced tens of thousands of people according to United Nations sources, many of whom head for camps in the area.

The families of Islamic State fighters have fled the U.S.-backed offensive, including for neighboring Deir al-Zor province, another IS stronghold, the Observatory says.

The U.S.-led coalition backs the SDF with air strikes and military advisors on the ground.