GENEVA -  Syria 's government met with United Nations envoys on Wednesday for talks aimed at ending the civil war, in a small boost for the negotiations that Damascus had threatened to boycott.

The government had initially refused to confirm it would attend the talks, which began on Tuesday, after the rebels signalled they would continue to insist on President Bashar al-Assad's ouster.

But a government delegation held initial meetings with UN mediator Staffan de Mistura in Geneva on Wednesday, reportedly after securing key concessions, including keeping the Assad issue off the table. With the help of Russian military support, the Syrian regime has made major advances against its opponents in the past two years, seizing back large chunks of the country and easing the pressure to negotiate.

The talks have achieved little through seven previous rounds but there are hopes the latest may make progress in ending the devastating conflict. De Mistura said the atmosphere was "constructive and professional" when he met the government at a luxury Geneva hotel earlier on Wednesday. He told reporters the talks may stretch into next week.

Opposition representatives, united in one delegation for the first time, met de Mistura on Tuesday.  They told the envoy they remained ready for face-to-face talks with the government. "We are one. We are ready to negotiate directly with the other side", opposition spokesman Yahya Aridi said in a statement.

De Mistura has said that he would push for direct talks once the opposition unified, but a source close to the government has told AFP that Damascus would not agree to sit around a table with rebel negotiators this round.

Rebel delegation chief Nasr al-Hariri had said that his camp was still insisting on Assad 's removal as part of any peace deal, defying calls for moderation. De Mistura had voiced hope the coming round would mark the first "real negotiation" on a possible deal to end the six-year war which has claimed more than 340,000 lives, forced millions to flee their homes and left Syria in ruin.

He has also warned the opposition that intransigence on the Assad issue might no longer be tenable. In September, he said the opposition needed to be "realistic" and accept that "they didn't win the war", a statement supported by facts on the ground.  With the help of Moscow, Assad 's government has regained control of 55 percent of the country. The rest is carved up between rebel factions, jihadists and Kurdish forces.

The decision last week by Syrian opposition groups to send a single delegation to Geneva raised hopes of a possible breakthrough.  The new rebel negotiating team includes members of the Saudi-backed High Negotiations Committee (HNC), which insists on Assad 's departure, as well as representatives of groups based in Moscow and Cairo that have a more moderate stance on the president.

Despite Hariri's firm public position on the Assad stalemate, a European diplomat said the situation was fluid.

"We expect (the opposition) will be pragmatic and flexible", the diplomat said, requesting anonymity. An opposition delegate, who requested anonymity, denied that his side was under pressure to abandon its hardline stance on Assad , calling such reports "absolutely untrue".

But a flexible opposition will likely help the UN's peace push, which has been overshadowed by negotiations spearheaded by Moscow.  Russia and its fellow ally Iran, along with rebel-backer Turkey, have hosted negotiations in the Kazakh capital of Astana that led to the creation of four "de-escalation zones" which produced a drop in violence.

Western powers are concerned that Russia is seeking to take a leading role in the peace process and will carve out a settlement that will largely favour Assad .