A Kurdish-led force backed by US-led air strikes has seized an Isis stronghold in northeastern Syria, a monitor has said.

The Syrian Democratic Forces were in full control of the town of Al-Shadadi in Hasakeh province on Friday, the Britain-based Syrian Observatory for Human Rights monitor told AFP.

The announcement follows earlier reports from AFP that Syrian Democratic Forces (SDF), backed by international strikes, were just three miles from Al-Shadadi.

The advance comes on the third day of a major offensive by the SDF - an alliance of Kurdish and Arab fighters.

The group has reportedly taken a series of villages and also cut two key Isis supply routes in the region, one from Al-Shadadi to Mosul in neighbouring Iraq, and a second from the town to Raqqa, the militant group's unofficial capital in Syria.

The Observatory said SDF fighters had also seized the Kibabeh oil field to the northeast of Al-Shadadi, after heavy fighting and multiple air strikes by the US-led coalition fighting Isis.

The SDF are also engaged in a major operation in Aleppo province, where they have taken key territory from rebel forces.