GENEVA - A delegation from Syria's main opposition group, the High Negotiations Committee, arrived at the United Nations in Geneva on Monday for a first official session in UN-mediated peace talks overseen by UN Syria envoy Staffan de Mistura.

The session is a prelude to the start of "proximity talks" that De Mistura hopes to hold between the opposition and a government delegation.

UN chief Ban Ki-moon on Monday urged Syria's warring parties gathered in Geneva for "overdue" UN-brokered peace talks to put their country's civilians above "partisan interests".

Ban, in an address to a defence college in Oman, also called on Gulf regional rivals Iran and Saudi Arabia, which support opposite sides in the war, to step up efforts to reach a compromise.

"We are all extremely glad these talks have started. They are long overdue," said the UN chief. "I urge all parties to put the people of Syria at the heart of their discussions, and above partisan interests. Civilians, including children and women, are bearing the brunt of this conflict," he said. "We must urgently see an end to the fighting, the sieges and the other terrible human rights abuses that have characterised this war."

In a shaky start to the talks, the UN envoy to Syria, Staffan de Mistura, was to hold separate talks Monday with representatives of the opposition and of President Bashar al-Assad's government.

He aims to coax the warring sides into six months of indirect "proximity talks" under an ambitious roadmap agreed by the many outside powers embroiled in the war that has killed more than 260,000 people.

Meanwhile, Syrian rebels fought back against an offensive by government forces near a supply route into the city of Aleppo on Monday and said there had been no letup in Russian air strikes.

The opposition to President Bashar al-Assad agreed late on Friday to travel to Geneva after saying they had received guarantees to improve the situation on the ground, such as a detainee release and a halt to attacks on civilian areas.

But the opposition says there has been no easing of the conflict since then, with government and allied forces including Iranian militias pressing offensives across important areas of western Syria, most recently north of Aleppo. It is the first big government offensive for nearly a year in the Aleppo area, which controls access to opposition-held areas of the city from Turkey, a sponsor of the insurgency.

"The attack started at 2 a.m., with air strikes and missiles," said rebel commander Ahmed al-Seoud, describing the situation near Aleppo, parts of which are controlled by the government and parts are in opposition hands.

Seoud told Reuters his Free Syrian Army group had sent reinforcements to the 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 Syrian Observatory for Human Rights 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 and Syrian state television said government forces were advancing.

The opposition High Negotiations Committee (HNC) has indicated it will leave Geneva unless steps set out in a Dec. 18 Security Council resolution, endorsing peace moves including a lifting of sieges of blockaded areas, are 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 before them.

Opposition delegate Farrah Atassi said government forces were escalating their military campaign, making it hard to justify the opposition's presence in Geneva. Another opposition official highlighted heavy Russian bombardment of the northern Homs area on Sunday as one sign that nothing had changed.

"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."

A senior Western diplomat said the opposition had shown up so as not to play "directly into the hands of the regime".

"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."

The HNC was due to meet De Mistura at 5 p.m. (1600 GMT) at the UN headquarters in Geneva.

It also met him at a hotel on Sunday where a diplomatic source said they discussed a proposal by De Mistura to help meet their humanitarian concerns.

The diplomatic efforts have been beset by difficulties, including a dispute over who should be invited to negotiate with the government. Beyond that, the challenges are enormous and include lingering divisions over Assad's future.

The Kurdish PYD party which controls large areas of northern Syria has been excluded in line with the wishes of Turkey, which considers it a terrorist group.

The HNC includes some of the foreign-backed rebel groups fighting Assad in western Syria. Islamic State is at war with both Assad and the rebels, and is fighting for its its own "caliphate" rather than a reformed Syria.

The United Nations is aiming for six months of talks that would focus on achieving a broad ceasefire, while also working towards a political settlement. The United States, which backs the opposition to Assad, has been urging the HNC to attend.

The Syrian government views all the groups fighting it as terrorists and instruments of foreign powers such as Saudi Arabia and Turkey.

Another complication is the type of government that will run Syria in any peace settlement. Jaafari said on Sunday Damascus favoured an "enlarged national government", while the opposition want a transitional governing body.