LONDON - Syrian government delegates are due to attend peace talks with opposition representatives Friday in Geneva to try to bring an end to five years of fighting that has killed more than half a million people. However, there are growing fears that the cease-fire which has underpinned the negotiations is about to break.

There's been an upsurge in fighting in Aleppo and Homs, where opposition supporters accuse the government of carrying out airstrikes on civilians.

The United Nations' Special Envoy to Syria — who is overseeing the talks — said humanitarian relief is struggling to get through to besieged towns.

"Everyone in the meeting was disappointed,” Staffan De Mistura told reporters Thursday. “Indeed, many of them are actually frustrated by the lack of new convoys reaching … some areas."

Those difficulties suggest the cease-fire is becoming increasingly shaky, says Ben Barry of the International Institute for Strategic Studies.

FILE - A convoy of trucks loaded with humanitarian
FILE - A convoy of trucks loaded with humanitarian supplies are seen heading to the besieged town of Madaya, 24 kilometers southwest of Damascus, Syria, for a U.N.-aid distribution, Jan. 14, 2016.

"The less the tension is on the frontlines, the easier it is for humanitarian aid convoys to pass through,” he said. “And as tension rises, it becomes more and more difficult. And put that together with evidence that the Syrian government appears to be gearing up for another major offensive around Aleppo."

‘No veto’ for opposition

The Syrian opposition — represented at the talks by umbrella group the High Negotiations Committee  — said Thursday it would accept a transitional government that includes current government figures.

"We will have no veto, as long as they don't send us criminals, as long as they don't send us people who are involved in the killing of Syrians," said the HNC's Salim Al-Muslat.

This photo released on the official Facebook page
This photo released on the official Facebook page of Syrian Presidency, shows Syrian President Bashar Assad casting his ballot in the parliamentary elections, as his wife Asma, left, is standing next to him, in Damascus, Syria, Wednesday, April 13, 2016.

The opposition insists Syrian President Bashar al-Assad cannot be part of the country's future.

But government negotiators, who will join the talks Friday, said the opposition is dreaming.

"Others need to forget the dreams they had for the last five years and to come with factual, actual solutions to the problem," said Faisal Mekdad, Syria's Deputy Foreign Minister.

But it's unclear exactly who is directing the Syrian government delegation, according to Barry.

"There has been evidence of different political factions jockeying for influence. And who exactly is writing what instructions for the Syrian delegate to the peace talks, I think is an important question," he said.

Meanwhile, the United States' ambassador to the U.N., Samantha Power, voiced her fears that the renewed fighting in Aleppo could derail the peace talks, and urged Assad's key ally, Russia, to "get the Syrian regime back with the program."

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