Beirut: Syria's landmark ceasefire was threatening to fall apart 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 27 February.
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 on Tuesday 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.
US concerns over Aleppo
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 Tuesday that continued indiscriminate attacks on civilians could cause the truce to break down.
It said recent attacks by rebel groups on Kurdish-majority neighbourhoods in Aleppo city and by government forces east of Damascus "threaten to derail the 'cessation of hostilities'."
"A decrease in casualty numbers brought a much-needed respite for Syrians, but many civilians are still dying in unlawful attacks," Nadim Houry, HRW's deputy Middle East director, said in a statement.
The ceasefire brought relative calm to swathes of Syrian territory in the north and centre, allowing increased humanitarian aid deliveries and a significant drop in daily deaths.
Syria's government and opposition will resume indirect peace negotiations on Wednesday in Geneva, with the fate of Assad still a major stumbling block.
The High Negotiations Committee, which represents Syria's main opposition groups, was due to arrive in Switzerland on Tuesday afternoon.
Russian helicopter crashes
The UN's De Mistura, who will host the talks, said the negotiations will focus on aspects of a peace roadmap calling for a transitional government, a new constitution and eventual elections.
"We will be focusing in particular on the political transition, on governance and constitutional principles," he told reporters in Damascus on Monday.
Despite the talks, Syria's regime will be going ahead on Wednesday with parliamentary elections taking place in government-controlled areas.
The United Nations does not recognise the vote and it has been dismissed by Assad's foreign and Syrian opponents as illegitimate.
De Mistura travelled from Syria to Iran on Tuesday to meet with senior officials in Tehran, which along with Moscow is one of Assad's key international backers.
As well as providing economic aid, Iran has sent military advisers from its elite Revolutionary Guards to Syria, dozens of whom have been killed.
Moscow launched a wave of air strikes in support of the regime last September, though last month Moscow ordered the bulk of Russian forces to withdraw.
Moscow's defence ministry said two Russian military pilots were killed in a helicopter crash near the central Syrian city of Homs on Tuesday.
It said the Mi-28 attack helicopter "was not fired at" and that the bodies were transported to the Hmeimim air base in northwest Syria.
Other world powers, including Turkey, have been drawn in to Syria's complex war and on Tuesday Turkish artillery hit IS targets across the border after rockets landed in the frontier town of Kilis, an official told AFP.