Syrian regime forces on Friday battled Islamic State group near Aleppo city as they clashed with a local Al-Qaeda affiliate and allied rebels nearby, a monitoring group said.

The upsurge in fighting, which the United States says is straining a fragile truce, came as a new round of peace talks got under way in Geneva.

"Fierce fighting raged between regime troops and loyalist militia against IS... to the east of Handarat" north of Aleppo city, the Syrian Observatory for Human Rights said.

The army is trying to recapture an area seized by IS on Thursday near Handarat, which lies along a route leading north out of opposition-held parts of Aleppo, the Britain-based monitor said.

Meanwhile, troops and militia loyal to President Bashar al-Assad's regime battled Syria's Al-Qaeda affiliate Al-Nusra Front and allied rebels on a northern front in the battered province, the group said.

Syrian warplanes carried out several strikes against Al-Nusra and rebel positions, as shells targeting the loyalists left six pro-regime militia fighters dead, it added.

The latest violence came a day after a senior official in Washington told AFP that the United States was "very concerned" about reports of a Russian-backed Syrian government offensive near Aleppo.

Even though the fight against IS and Al-Nusra is excluded from the truce, violence around Aleppo has sparked concerns that the ceasefire may not last, partly because rebels are involved in the battles there too.

"Aleppo is the key to war and peace in Syria," Observatory director Rami Abdel Rahman told AFP.

"Every side in the war has a stake in Aleppo."

Syria's conflict began in 2011 as a peaceful revolt, with protests across the country that spread in 2012 to Aleppo province, which borders Turkey.