By Tom Perry, Suleiman Al-Khalidi and John Irish

BEIRUT/AMMAN/GENEVA (Reuters) - A Syrian military offensive backed by heavy Russian air strikes threatened to cut critical rebel supply lines into the northern city of Aleppo on Tuesday, while the warring sides said peace talks had not started despite a U.N. statement they had.

U.N. 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 militants.

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

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. We must absolutely try to ensure that there is no failure," he told Swiss television RTS.

The opposition canceled a meeting with him on Tuesday afternoon, 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.

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

The main Syrian opposition council said after meeting de Mistura on Monday it had not, and would not negotiate unless the government stopped bombarding civilian areas, lifted blockades on besieged towns and released detainees.

Conditions are dire in a number of areas under siege by both sides, with many close to starvation. However, the Syrian Red Crescent delivered 14 truckloads of aid to the town of al-Tal north of Damascus on Tuesday, in an area surrounded by forces allied to the government.

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SLIM CHANCES

The head of the Syrian government delegation also denied talks had started after discussions with de Mistura on Tuesday.

Bashar al-Ja'afari said after two and a half hours of talks that the envoy had yet to provide an agenda or list of opposition participants. "The formalities are not yet ready," he told reporters at the United Nations office in Geneva.

He also said that if the opposition "really cared" about the lives of Syrians it should condemn the killing of more than 60 people on Sunday by Islamic State bombers in a neighborhood that is home to the country's holiest Shi'ite shrine.

A U.N. source said de Mistura had promised to present an opposition delegation list by Wednesday. Its makeup is subject to fierce disagreements among the regional and global powers that have been drawn into the conflict.

The refugee crisis and spread of the jihadist Islamic State through large areas of Syria, and from there to Iraq, has injected a new urgency to resolve the five-year-old Syria war.

But the chances of success, always very slim, appear to be receding as the government, supported by Russian air strikes, advances against rebels, some of them U.S.-backed.

U.S. Secretary of State John Kerry called on Moscow to stop the bombing during the peace process. "We are beginning the talks, we are at the table and we expect a ceasefire," he said after a meeting in Rome of countries opposed to Islamic State.

"DECISIVE BATTLE"

The attack north of Aleppo that began in recent days is the first major government offensive there since the Russian air strikes began on Sept. 30.

The area safeguards a rebel supply route from Turkey into opposition-held parts of the city and stands between government-held parts of western Aleppo and the Shi'ite villages of Nubul and al-Zahraa which are loyal to Damascus.

"The supply routes were not cut but there is heavy bombardment of them by the jets," said a commander in the Levant Front rebel group who gave his name as Abu Yasine. "The Russian jets are trying to hit headquarters and cut supply routes."

The Russian jets had been working "night and day" for three days, he added, and reiterated the rebels' long-held demand for anti-aircraft missiles to confront the assault.

"If there is no support, the regime could besiege the city of Aleppo and cut the road to the north," said Abu Yasine, whose group is one of the rebel movements that have received military support from states opposed to Assad, funneled via Turkey.

Advancing government forces seized the village of Hardatnin some 10 km (six miles) northwest of Aleppo, building on gains of the previous day, said the Syrian Observatory for Human Rights, a UK-based monitoring body.

Another rebel commander said he had sent reinforcements to the area. "We sent new fighters this morning, we sent heavier equipment there. It seems it will be a decisive battle in the north, God willing," said Ahmed al-Seoud, head of a Free Syrian Army group known as Division 13. "We sent TOW missile platforms. We sent everything there," he told Reuters.

U.S.-made TOW missiles, or guided anti-tank missiles, are the most potent weapon in the rebel arsenal and have been supplied to vetted rebel groups as part of a program of military support overseen by the Central Intelligence Agency.

But while they have helped rebels to slow advances on the ground, they are of little use against fighter bombers.

The Russian intervention has reversed the course of the war for Damascus, which suffered a series of major defeats to rebels in western Syria last year before Moscow deployed its air force as part of an alliance with Iran.

In an interview with Reuters, British Foreign Secretary Philip Hammond said Russian President Vladimir Putin was undermining international efforts to end the war by bombing opponents of Islamic State in an attempt to bolster Assad.

"The Russians say let's talk, and then they talk and they talk and they talk. The problem with the Russians is while they are talking they are bombing, and they are supporting Assad," Hammond said.

Russia's Foreign Ministry said Hammond was spreading "dangerous disinformation", while the Kremlin said his statements could not be taken seriously.

(Additional reporting by John Davison in Beirut, Stephanie Nebehay and Kinda Makieh in Geneva and Crispian Balmer in Rome; writing by Tom Perry; editing by Philippa Fletcher, Peter Graff and David Stamp)