Fighters from the Syrian Democratic Forces (SDF), an Arab-Kurdish alliance backed by the United States, have advanced to the edge of Tabqa, a key jihadist-held town in northern Syria, the Syrian Observatory for Human Rights said Saturday.

SDF fighters “are now hundreds of meters from Tabqa”, said Rami Abdel Rahman, head of the Observatory, a British-based monitoring group.

The alliance, which is supported by US-led coalition air strikes and special forces advisers, surrounded Tabqa in early April.

The town and nearby major dam are considered a key prize in a broader offensive for Raqqa, ISIS’ main base of operations in Syria, about 55 kilometers to the east.

The alliance was reported to have advanced on Tabqa overnight after driving the jihadists from two areas just southeast and southwest of the town.

“Heavy fighting is taking place in the vicinity of the two suburbs,” Abdel Rahman said. “ISIS is trying to counter-attack.”

The terrorist organization is under pressure on several fronts, with regime forces attacking it elsewhere in Syria and a US-backed offensive targeting its Iraqi stronghold of Mosul.

The Pentagon said on Thursday that a US-led air strike mistakenly killed 18 SDF members south of Tabqa.

The US-led coalition forces struck the position on Tuesday after another partner in the fight wrongly told them it was occupied by ISIS jihadists, the Pentagon said, underlining the complex nature of the conflict.

The SDF launched its campaign for Raqqa in November and has since captured most of the surrounding province.