GENEVA/BEIRUT/ROME (REUTERS) - The United Nations announced the formal start of peace talks for Syria on Monday and urged world powers to push for a ceasefire even as government forces, backed by Russian air strikes, launched their biggest offensive north of 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. The Syrian people deserved to see improvements on the ground and the opposition had a "strong point" in demanding goodwill steps, he said.

World powers, he said, should immediately begin talks on how to enforce a ceasefire: "There was a message ... that when the Geneva talks actually start, in parallel there should be the beginning of a serious discussion about ceasefires."

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.

A senior US official returned from a fact-finding visit to northern Syrian territory held by Kurdish fighters, who have advanced against ISIS (Islamic State in Iraq and Syria) militants with the help of US air support.

Opposition delegates agreed late on Friday to travel to Geneva after saying they had received guarantees to improve the situation on the ground. 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.

"The (latest) attack started at 2 a.m., with air strikes 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 said his Free Syrian Army group had sent reinforcements to an area near the village of Bashkoy.

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 air strikes on Monday morning. Syrian state television also said government forces were advancing.

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 death toll from an ISIS suicide attack near Damascus on Sunday climbed to more than 70 people, the Observatory said.

The opposition High Negotiations Committee (HNC) indicated it would leave Geneva unless peace moves were implemented.

Bashar al-Jaafari, 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 to begin them.

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 U.N. 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. 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 ISIS, also known as 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.

Brett McGurk, US envoy to the coalition, said he had visited territory held by Kurdish fighters in Syria over the weekend to assess the counter-Islamic State campaign.

The Kurds have proven the most capable allies of U.S.-led forces on the ground in Syria. But their relationship with Washington irks U.S. ally Turkey, which sees the Syrian Kurds as allies of its own Kurdish separatist militants. The Syrian Kurds have so far been excluded from the Geneva talks.

McGurk said he had discussed next steps in the Syria campaign with "battle-tested and multi-ethnic anti-ISIL fighters", and Washington supported an inclusive approach to the talks.

All previous diplomatic efforts have failed to stop the war.

A senior Western diplomat said the opposition had shown up in the Swiss city so as not to play "into the hands of the regime" by staying away.

"They want tangible and visible things straight away, but there are things that realistically can't be done now such as ending the bombing. It's obvious that that is too difficult. The easiest compromises are releasing civilians and children."