Syria's landmark ceasefire was threatening to fall apart after a surge of fresh fighting, especially in northern Aleppo province, just as peace talks were set to resume in Geneva on Wednesday.

The UN's Syria envoy Staffan de Mistura, who has said the negotiations will 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 will be the second since the Assad government 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 ending a five-year-old conflict that has left more than 270,000 dead and forced nearly half of the country’s population from their homes.

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De Mistura, who will host the talks, said the negotiations would focus on aspects of a peace roadmap that calls for a transitional government, a new constitution and, eventually, elections.

But the fate of Assad is still a major stumbling block.

"We will be focusing in particular on the political transition, on governance and constitutional principles," he told reporters in Damascus on Monday.

But concern has been mounting that a spike 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 Britain-based Syrian Observatory for Human Rights monitoring group said.

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Al-Nusra and the Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) were excluded from the ceasefire but, complicating matters, in some areas the al-Qaeda fighters are allied with rebels covered by the truce.

Government planes carried out "unprecedented" air strikes in recent days on the rebel-held eastern parts of Aleppo city, according to the Observatory, which relies on a broad network of sources inside Syria.

Government forces, backed by Russian air power, pressed a similar offensive around Aleppo city during a previous failed round of talks at the end of January. Western powers blamed that escalation for the breakdown of those talks.

Al-Nusra and allied rebel groups were also 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.

Wave of strikes

The New York-based Human Rights Watch (HRW) has also warned 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 parts of northern and central Syria, allowing increased humanitarian aid deliveries and a significant drop in daily deaths.

Despite the talks, the government will go ahead on Wednesday with parliamentary elections in the areas it controls.

The UN 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 government last September, though last month Moscow ordered the bulk of Russian forces to withdraw.

Russia’s defence ministry said two Russian military pilots were killed in a helicopter crash near the central Syrian city of Homs on Tuesday.