GENEVA/MUSCAT • The United Nations (UN) strained to keep Syrian peace talks alive yesterday as Damascus tried to press home gains against rebels, and its ally Russia said its air strikes would go on until "terrorists" were defeated.

UN envoy Staffan de Mistura announced the formal start on Monday of the first attempt in two years to negotiate an end to a war that has killed 250,000 people, caused a refugee crisis in the region and Europe and empowered Islamic State in Iraq and Syria militants.

But both opposition and government representatives have since said that talks have not in fact begun, and fighting on the ground has raged on without constraint.

The opposition cancelled a meeting with Mr de Mistura on Tuesday, and issued a statement condemning "a massive acceleration of Russian and regime military aggression on Aleppo and Homs", calling it a threat to the political process.

Mr de Mistura acknowledged that a collapse of the Geneva talks was always possible.

"If there is a failure this time after we tried twice at conferences in Geneva, for Syria, there will be no more hope," he told Swiss television channel RTS.

Rebels described the assault north of Aleppo as the most intense yet. One commander said that opposition-held areas of the divided city were at risk of being encircled entirely by the government and allied militia. He appealed to foreign states that back the rebels to send more weapons.

"The Russians and regime want to push the opposition out of Geneva so the opposition bears the responsibility for the failure," said a senior Western diplomat.

Despite calls from the United States and its allies for Moscow to stop the bombing during the peace process, Russian Foreign Minister Sergei Lavrov said that his country has no intention of ending its campaign.

"Russian strikes will not cease until we really defeat terrorist organisations like Jabhat al-Nusra. And I don't see why these air strikes should be stopped," he said at a news conference yesterday in Oman's capital, Muscat.

The opposition tentatively said that it would resume meetings with Mr de Mistura yesterday.

Its chief coordinator Riad Hijab, who diplomats say is a unifying figure for the fragmented opposition, was expected to arrive in Geneva later yesterday.

The attack north of Aleppo that began in recent days is the first major government offensive there since Russian air strikes began on Sept 30. The area safeguards a rebel supply route from Turkey into the opposition-held parts of the city.

NEW YORK TIMES, REUTERS, AGENCE FRANCE-PRESSE