Over 100 troops, pro-regime militia, jihadists and rebels have been killed in four days of fierce fighting on a strategic front of Syria's Aleppo province, a monitoring group said today.

Since Sunday, fighting around Al-Eis and Khan Touman in Aleppo's southern belt has killed 61 rebels and members of Al-Qaeda-affiliated Al-Nusra Front and 50 troops and pro-regime militia, said the Syrian Observatory for Human Rights.

"In the past 24 hours alone, 42 rebels and Al-Nusra members died, as well as 34 regime loyalists," Observatory director Rami Abdel Rahman said.

Regime troops are trying to recapture Al-Eis, held by Al-Nusra and rebel allies, which in turn have launched an offensive to take over nearby Khan Touman from the regime.

The fighting came as UN-brokered indirect talks resumed in Geneva, threatening to break a fragile six-week truce that was brokered by the United States and Russia.

Neither Al-Nusra nor the jihadist Islamic State group are included in the truce, 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 more moderate factions, and cause the truce to collapse and derail the peace efforts.

The area where the fighting is focused is important because it is located near the highway linking Damascus to war-ravaged Aleppo city, the Observatory said.

It is also key because it is near the Shiite towns of Fuaa and Kefraya in neighbouring Idlib province, which are under siege by opposition forces.

"Most of the regime loyalists killed were militia fighters from Syria, Lebanon, Iraq, Iran and Afghanistan," Abdel Rahman said.

"For them, this is an ideologically-driven battle to break the siege on Fua and Kefraya," he told AFP.

Abdel Rahman said the fighting shows that neither President Bashar al-Assad's regime nor the opposition represented at the Geneva talks calls the shots in fighting on the ground.

"The real decisions are made by (regime backers) Iran and Russia on one side, and jihadist factions and opposition backers on the other," he said.

Syria's war began as a popular anti-regime revolt but later morphed into a brutal civil war after Damascus unleashed a brutal crackdown on dissent.