IS captures opposition-held Syrian villages near Turkish border

"Fierce clashes are raging between rebels and IS after the jihadists secured an advance and seized control of six villages near the Turkish border," reported the Syrian Observatory for Human Rights.

The most important village to come under IS control is Hiwar Kallis, about a kilometre south of the Turkish border, said Observatory director Rami Abdel Rahman.

Military aircraft meanwhile raided IS positions in the area, he said, adding that they were likely from the US-led coalition.

In recent weeks, battles have been intensifying between IS and rebels over the strip along the border, which both sides use to smuggle fighters and weapons.

Rebels had secured significant gains against IS before the jihadists pushed them back, including in the strategic town of Al-Rai, which is on an IS supply route to Turkey.

Meanwhile, rebels allied with the al-Qaeda-affiliated Nusra Front fought fresh clashes against regime troops and allied militias in southern Aleppo, the Observatory said.

More than 100 fighters from across both sides have been killed in fighting there since Sunday, the UK-based monitor said.

Neither Nusra nor IS are included in the truce that began on February 27, but the fact that some rebels are fighting alongside Nusra while regime forces push back has sparked concerns over the ceasefire's durability.

Washington voiced concern on Monday that a regime assault on the Nusra Front in Aleppo could spread to areas controlled by moderate rebel factions, and cause the truce to collapse and derail peace efforts.

Syria's conflict began in 2011 as a peaceful revolt seeking democratic change, but has since escalated into a multi-front civil war that has left more than 270,000 people dead and forced millions to flee their homes.