Islamic State (ISIS) jihadists have seized a string of opposition-held villages in Syria's Aleppo province near the Turkish border, a monitoring group said Thursday.
Areas around Aleppo have seen a spike in violence on several fronts in recent days, sparking concern over a fragile six-week truce as a new round of peace talks gets under way in Geneva.
"Fierce clashes are raging between rebels and (ISIS) after the jihadists secured an advance and seized control of six villages near the Turkish border," the Syrian Observatory for Human Rights said.
The most important village to come under ISIS control was Hiwar Kallis, about one kilometer (less than one mile) south of the Turkish border, Observatory director Rami Abdel Rahman said.
Warplanes meanwhile raided ISIS positions in the area, he said, adding that they were likely from the US-led coalition.
In recent weeks, battles have been intensifying between ISIS and rebels over the border strip, which both sides use to transport fighters and weapons.
Rebels had secured significant gains against ISIS before the jihadists pushed them back, like in the strategic town of Al-Rai, which is on an ISIS supply route to Turkey.
Meanwhile, rebels and their allies from the Al-Qaeda-affiliated Al-Nusra Front fought fresh clashes against regime troops and militia in southern Aleppo, the Observatory said.
Over 100 fighters from both sides have been killed in fighting there since Sunday, the Britain-based monitor said.
Neither Al-Nusra nor ISIS are included in the truce that began on February 27, but the fact that rebels are fighting alongside Al-Nusra while regime forces push back has sparked concerns over its durability.
Washington voiced concern Monday that a regime assault on Al-Nusra 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 over 270,000 people dead and forced millions to flee their homes.
AFP contributed this report.