A Kurdish militia in northern Syria has joined forces with Arab rebels, and their new alliance has been promised fresh weapon supplies by the United States for an assault on Islamic State forces in Raqqa, a spokesman said yesterday.

The alliance calling itself the Democratic Forces of Syria includes the Kurdish YPG militia and Syrian Arab groups, some of which fought alongside it in a campaign that drove Islamic State from wide areas of northern Syria earlier this year.

The Arab groups in the new alliance are operating under the name “The Syrian Arab Coalition” - a grouping which US officials have said would receive support under a new US strategy aimed at fighting Islamic State in Syria.

A spokesman for some of the Arab rebels said they were told by Washington that new weapons were being sent to help them launch a joint offensive on the city of Raqqa.

“We met the Americans and this has been approved and we have been told these new arms... are on their way,” said Abu Muazz, a spokesman for the Raqqa Revolutionaries Front, a grouping of mainly Arab tribal insurgents who are mostly drawn from the Raqqa area. He said the group constituted a 3,200-strong, well trained fighting force which could begin using the weapons within days of their arrival. It has an additional 600 fighters who are currently wounded, he said.

Groups aim at fighting Islamic State in Syria

A major offensive against the ultra-hardline Islamic State fighters could capture Raqqa in less than two months provided the “right weapons and quantities” arrived, Abu Muazz, himself from the jihadist-held city, said.

A US military official said that the Syrian Arab Coalition would push down towards Raqqa, Islamic State’s de facto capital, while staying east of the Euphrates river.

Keeping the YPG-backed force east of the river could ease Turkish concerns about any further expansion of Kurdish influence in northern Syria. Turkey is worried about the Kurds’ growing power in Syria fuelling separatism among its own Kurds. The US last week announced a shake-up of its support to Syrian rebels fighting IS, effectively ending its programme to train fighters outside Syria and focusing instead on providing weapons to groups whose commanders have been US-vetted. The YPG has to date proved the most effective partner on the ground for US-led air strikes against Islamic State.