Beirut: Syria's landmark ceasefire was threatening to fall apart on Tuesday amid a surge of fresh fighting, especially in northern Aleppo province, just as peace talks were set to resume in Geneva.
The UN's Syria envoy Staffan de Mistura, who has said the negotiations due to start Wednesday would be "crucially important," was in Iran for talks with a key backer of Syria's President Bashar al-Assad.
This week's round of talks in Geneva will be the second since Assad's regime and rebel forces agreed to a partial truce brokered by Moscow and Washington, which has largely held since February 27.
It has raised hopes that steps may finally be taken towards resolving the five-year-old conflict, which has devastated the country and left more than 270,000 dead.
But concern has been growing that a recent rise in violence focused mainly in Aleppo province, which borders Turkey, is putting intense strain on the ceasefire.
Pro-government forces were today pressing an advance against the town of Al-Eis, held by fighters from al-Qaeda's local affiliate, Al-Nusra Front, and allied rebels, the Syrian Observatory for Human Rights monitoring group said.
Jihadists like those from Al-Nusra and the Islamic State group are excluded from the ceasefire but in some areas the al Qaeda militants are allied with rebel forces meant to be covered by the truce.
Regime warplanes have also carried out "unprecedented" air strikes in recent days on the rebel-held eastern parts of Aleppo city, according to the Britain-based Observatory, which relies on a broad network of sources inside Syria.
Russian-backed regime forces pressed a similar offensive around Aleppo city during a previous failed round of peace talks at the end of January.
Western powers blamed the government's military escalation for the breakdown of those talks.
Al-Nusra and allied rebel groups were meanwhile pushing their own offensive on the town of Khan Touman near Aleppo city, the Observatory said.
Washington has expressed worries that an assault against Al-Nusra in Aleppo may spread to moderate rebel factions, which could cause the truce to collapse and derail peace efforts.
"We are concerned about plans to attack and seize ... Aleppo when there are clearly opposition groups there that are part of the cessation of hostilities," State Department spokesman Mark Toner told reporters on Monday.
Human Rights Watch also warned that continued indiscriminate attacks on civilians could cause the truce to break down.