The Kurds, aided by US-led air strikes and smaller Syrian rebel groups, pushed on Monday to within 7 km (4 miles) of Ain Issa, a town 50 km (30 miles) north of Islamic State's de facto capital Raqqa city, said Redur Xelil, a spokesman for the Kurdish forces.

"They have been defeated," YPG spokesman Redur Xelil told reporters. Islamic State had held the military base, Liwa 93, southwest of Ain Issa, since capturing it from the Syrian military last year.

"This means that the Islamic State keeps collapsing inside its own stronghold," stated Rami Abdulrahman, head of the Syrian Observatory for Human Rights, which monitors the conflict.

The rapid advance into Raqqa province has defied expectations of a protracted battle between the Kurdish YPG group and Islamic State fighters, who waged a four-month battle for the border town of Kobani, where the Kurds finally defeated the jihadists in January.

Raqqa is the main seat of power in Syria for Islamic State, the group also known as ISIS or ISIL, which has proclaimed a caliphate to rule over all Muslims from territory it controls in Syria and Iraq.

The United States has been leading an air campaign against the group in both countries since last year. The Kurds have been the most important partner so far for the US-led campaign in Syria, where Washington has far fewer allies on the ground than in Iraq.

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