Syria's main opposition group agreed to travel to Geneva, where the United Nations on Friday opened peace talks to end the country's five-year-old war, but said it wanted to discuss humanitarian issues before engaging in political negotiations.

On the ground, opponents of President Bashar al-Assad said they were facing a Russian-backed military onslaught, with hundreds of civilians reported to be fleeing as the Syrian army and allied militia tried to capture a suburb of Damascus and finish off rebels defending it.

UN envoy Staffan de Mistura had invited the Syrian government and an opposition umbrella group to Geneva for "proximity talks", in which they would meet in separate rooms.

Until the last minute, the opposition High Negotiations Committee (HNC) had refused to go. The group, which includes both armed and political opponents of Assad, had insisted it wanted an end to air strikes and sieges of towns and the release of detainees before talks could start.

Late on Friday, the HNC said it was going to Geneva, having received guarantees that its demands, outlined in a UN Security Council resolution last month, would be met, but it made clear its engagement in the process would initially be limited.

"The HNC will go to Geneva tomorrow to discuss these humanitarian issues which will pave the way into the political process of negotiations," spokesman Salim al-Muslat told the Arabic news channel al-Arabiya al-Hadath.

The HNC said it had drawn up a list of 3,000 Syrian women and children in government prisons who should be released.

Sunday meeting

De Mistura opened the talks on Friday by meeting the Syrian government delegation. He said that while he had not yet received formal notice that the HNC would attend, he expected to meet its delegation on Sunday.

"They've raised an important point of their concern, they would like to see a gesture from the government authorities regarding some kind of improvement for the people of Syria during the talks, for instance release of prisoners, or some lifting of sieges," de Mistura said.

But he added this was a human rights point and "not even an issue to negotiate", and had strongly suggested the best way to get such measures implemented would be to start negotiating in Geneva, by proxy or directly.

US Secretary of State John Kerry had made a major push to get the HNC delegation to Geneva, and the group said he had contacted it by phone to urge it to attend.

"Secretary Kerry has been in touch with all of his counterparts, including this morning with (Russian Foreign Minister) Sergei Lavrov ... and with others, trying to find a way, a formula, in which we can urge the delegation or some version of the delegation to show up here," a senior US official said.

The Syrian government delegation, headed by United Nations ambassador Bashar al Jaafari, arrived at the talks on Friday afternoon but made no statement.

Another major force, the Kurds who control much of northeast Syria and have proven one of the few groups capable of winning territory from Islamic State, were excluded from the talks after Turkey demanded they be kept away. The Kurds say their absence means the talks are doomed to fail.

Government momentum

International diplomacy has so far seen only failures in a 5-year-old multi-sided ethno-sectarian civil war that has killed more than 250,000 people and driven more than 10 million from their homes while drawing in regional states and global powers.

De Mistura's two predecessors both quit in apparent frustration after staging failed peace conferences.

Since the last talks collapsed in 2014, Islamic State fighters surged across Syria and Iraq declaring a "caliphate", the United States and its European and Arab allies launched air strikes against them, and Russia joined in last year with a separate air campaign to support Assad.

Moscow's intervention in particular has altered the balance of power on the ground, giving strong momentum to government forces and reversing months of rebel gains.

The Syrian military and allied militia are seeking to build on gains in western Syria, and have turned their focus to opposition-held suburbs southwest of Damascus.

The aim is to crush rebels in the district of Daraya to secure the nearby military airport at Mezzeh, said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, which monitors the conflict with sources on the ground.

Rebels had rejected a government deadline for them to withdraw from the suburb of Mouadamiya - home to 45,000 people - by Friday, said Abu Ghiath al-Shami, spokesman for rebel group Alwiyat Seif al-Sham.

More than 500 families had fled, he said. "They are suffering a shortage of food, medicine, milk - there is no power, nothing," he said, adding that 16 barrel bombs had been dropped on Friday.

A Syrian military source denied the use of barrel bombs, which have been widely documented in the war, and accused the opposition of exaggerating the conditions. "There has been progress by the army in the last days, some successes particularly in the Daraya area," the source said.

Rebels say the fighting is of more concern to them than the fate of the negotiations. Asked about the future of the talks, Shami told Reuters he was "a bit busy" dealing with the fighting.