The strikes are some of the most sustained since they began in September, according to US officials who say they are aimed at curbing the militants` ability to operate out of Raqqa and to prevent it from fighting back against Kurdish advances.
But, on Tuesday, the ultra-hardline group was still in control of Ain Issa, the Observatory said. The town, 50 km (30 miles) north of Raqqa city, was seized by Islamic State fighters from the Kurdish YPG militia in an attack on Monday.
That attack on YPG-held areas followed an intensification of air strikes on Raqqa city over the weekend, which US defense secretary Ash Carter said on 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 US-led alliance fighting to tackle Islamic State both there and in Iraq.
The Observatory, a British-based organisation 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 Islamic State in Raqqa province in recent weeks, seizing Tel 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 Islamic State, 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 Islamic State in Syria. But the Pentagon said on June 18 the effort was moving more slowly than expected.
US president Barack Obama said on Monday the United States would do more to train and equip "the moderate opposition".
More about:
