The United Nations announced on Monday peace talks for Syria had begun and called on world powers to push for a ceasefire, even as government forces, backed by Russian air strikes, launched their biggest offensive near Aleppo in a year.

Government troops and allied fighters captured hilly countryside near Aleppo on Monday, putting a key supply route used by opposition forces into firing range, according to the Syrian Observatory for Human Rights monitoring group.

Rebels said the offensive was being conducted with massive Russian air support, despite a promise of goodwill steps by the Syrian government to spur peace negotiations. The opposition has said that without a halt to bombing, the lifting of sieges on towns and freeing of prisoners, it will not participate in talks in Geneva called by the United Nations.

Nevertheless, opposition delegates met in Geneva for two hours with UN envoy Staffan de Mistura, who said this session marked the official beginning of peace talks. He also said that the Syrian people deserved to see improvements on the ground and the opposition had a "strong point" in demanding goodwill steps.

International powers should immediately begin talks on how to enforce a ceasefire, he said.

1st attempt at negotiations in 2 years

The government's military assault has overshadowed de Mistura's attempts to convene the first peace negotiations in two years, intended to start as "proximity talks," with government and opposition delegations in separate rooms.

A senior U.S. official returned from a visit to northern Syrian territory held by Kurdish fighters, who have advanced against Islamic State militants with the help of U.S. air support. The Geneva peace talks mark the first attempt in two years to hold negotiations to end a war that has drawn in regional and international powers, killed at least 250,000 people and forced 10 million from their homes.

The death toll from an Islamic State suicide attack near Damascus climbed to more than 70 people, the Observatory said. The attack by the Sunni Muslim militants targeted a government-held neighbourhood that is home to Syria's holiest Shia Muslim shrine.

Opposition delegates agreed late on Friday to travel to Geneva after saying they had received guarantees to improve the situation on the ground, such as a release of detainees and a halt to attacks on civilian areas. But the opposition says there has been no easing of the conflict, with government and allied forces including Iranian militias pressing offensives across important areas of western Syria, most recently north of Aleppo.

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Wounded Syrians wait for assistance at a makeshift hospital in the rebel-held Damascus suburb of Douma following a reported airstrike by government forces on the town, 13 kilometres northeast of the Syrian capital on Nov. 11, 2014. The Russian air force reportedly carried out airstrikes on Douma last month. (Abd Doumany/AFP/Getty Images)

"The [latest] attack started at 2 a.m., with airstrikes and missiles," said rebel commander Ahmed al-Seoud, describing the situation near Aleppo, once Syria's biggest city and commercial centre, now partly ruined and divided between government and insurgent control.

Seoud told Reuters his Free Syrian Army group had sent reinforcements to an area near the village of Bashkoy.

"We took guarantees from America and Saudi to enter the negotiations ... [but] the regime has no goodwill and has not shown us any goodwill," he said from nearby Idlib province.

The British-based Observatory monitoring group said government forces were gaining ground in the area, and had captured most of the village of Duweir al-Zeitun near Bashkoy.

It reported dozens of airstrikes on Monday morning. Syrian state television also said government forces were advancing.

Humanitarian crisis worsens

The fighting has created a new flow of refugees.

A Turkish disaster agency said more than 3,600 Turkmens and Arabs fleeing advancing pro-government forces in northern Latakia province had crossed into Turkey in the past four days.

The opposition High Negotiations Committee (HNC) indicated it would leave Geneva unless peace moves were implemented. Bashar al-Jaafari, the head of the government delegation, said on Sunday Damascus was considering options such as ceasefires, humanitarian corridors and prisoner releases.

But he suggested they might come about as a result of the talks, not as a condition for negotiations to begin.

The humanitarian crisis wrought by the almost five-year-old conflict has worsened as a result of the increased fighting. International attention has focused in particular on the fate of civilians trapped and starving in besieged towns.

The United Nations said on Monday the Syrian government had approved "in principle" a UN request for aid deliveries to the town of Madaya, under siege from government forces, as well as the towns of al-Foua and Kefraya, beset by insurgents.

"Based on this, the UN will submit a detailed list of supplies and other details; and will include and reiterate the request for nutrition supplies and entry of nutrition/health assessment teams," said Jenes Laerke, spokesman for the UN Office for the Coordination of Humanitarian Affairs. No date was given for aid shipments.

Opposition delegate Farrah Atassi said government forces were escalating their military campaign, making it hard to justify the opposition's presence in Geneva.

"Today, we are going to Mr De Mistura to demand again and again, for a thousand times, that the Syrian opposition is keen to end the suffering of the Syrian people," Atassi said. "However, we cannot ask the Syrian opposition to engage in any negotiation with the regime under this escalation."

Since the last Syrian peace talks took place in early 2014, militants from Islamic State, also known as ISIS, ISIL or Daesh, have proclaimed a "caliphate" in swathes of Syria and Iraq, drawing a U.S.-led coalition into the conflict with air strikes.

Last year Russia began a separate campaign of air strikes in support of its ally, President Bashar al-Assad.