Kerry warns Russia over 'frayed' Syria ceasefire

US Secretary of State John Kerry yesterday warned Russia and the Syrian government to respect a "frayed" ceasefire, as vicious fighting south of second city Aleppo left dozens dead.

The vital northern province of Aleppo has been ravaged on multiple fighting fronts in a devastating war that has killed more than 280,000 people.

"Russia needs to understand that our patience is not infinite, in fact it is very limited with whether or not Assad is going to be held accountable," said Kerry.

Washington and Moscow have tried to reinforce the broader truce with temporary, local freezes on fighting, but to no avail.

"It is very clear that the cessation of hostilities is frayed and at risk and that it is critical for a genuine cessation to be put in place. We know that, we have no illusion," said Kerry.

Efforts were underway, he said, to reach a new agreement "in the next week or two" to reinstate the ceasefire across Syria, leading to more humanitarian aid deliveries and a resumption of the peace process.

In Aleppo province, dozens of fighters were killed in a fresh bout of fighting between the regime, rebels, and jihadists south of Aleppo city, said the Syrian Observatory for Human Rights.

Since fighting erupted on Tuesday, at least 70 fighters in total have been killed and the villages of Zaytan and Khalasa have changed hands twice.

Meanwhile, forces allied with Libya's unity government said yesterday they had repelled a counter-attack by the Islamic State group in the jihadists' coastal stronghold of Sirte.

IS jihadists hit positions controlled by fighters loyal to Libya's UN-backed Government of National Accord west of the city with tank, mortar and sniper fire on Tuesday.

The Islamic State group battled Iraqi forces and held civilians hostage yesterday to defend its bastion of Fallujah.

Thousands of men trying to flee Fallujah were being held for screening by pro-government forces and allegations of abuses were mounting.

"Let's be absolutely clear: there is no safe passage out of Fallujah to speak of," NRC country director Nasr Muflahi said in a statement.