BEIRUT: Syrian Kurdish fighters have recaptured more than 10 villages seized by ISIS north of its de facto capital of Raqqa, aided by U.S.-led coalition air strikes, the activist group the Syrian Observatory for Human Rights reported Tuesday.

Fighters from the ultra-hardline Islamist group were, however, still in control of the town of Ain Issa, 50 km (30 miles), north of Raqqa city, which the jihadis seized from the YPG militia in an attack Monday, the Observatory reported.

That attack on YPG-held areas followed an intensification of airstrikes on Raqqa city over the weekend, which U.S. Defense Secretary Ash Carter said Monday were aimed at disrupting the militants' ability to respond to YPG advances north of Raqqa.

The YPG, a militia operating mostly in predominantly Kurdish areas of northern Syria towards the Turkish border, has emerged as the only significant partner in Syria for U.S.-led alliance fighting to tackle ISIS both there and in Iraq.

The Observatory, a British-based activist group reporting on Syria's four-year-old civil war, said the coalition had played an "effective role" in helping YPG forces recover 11 villages northeast of Ain Issa.

The YPG, backed by small Syrian rebel groups, has made significant gains against ISIS in Raqqa province in recent weeks, seizing Tal Abyad at the Turkish border on June 15 before advancing south to Ain Issa.

The YPG captured Ain Issa on June 23.

While the YPG has shown itself to be a potent force in the fight against ISIS, its effectiveness is seen to diminish beyond the predominantly Kurdish areas it was set up to defend in northern and northeastern Syria.

The United States aims to train and equip Syrian rebel fighters it deems politically moderate in order to fight ISIS in Syria. But the Pentagon said on June 18 the effort was moving more slowly than expected.

U.S. President Barack Obama said Monday the United States would do more to train and equip "the moderate opposition."