GENEVA (AP) – The U.N. special envoy for Syria announced the official start of peace talks Monday between the Syrian government and opposition leaders, hoping to keep a wobbly process alive and compel world powers who helped set the stage for the talks to do more to bring about a cease-fire in a five-year war.

Staffan di Mistura said the mere arrival of a delegation from the main Syrian opposition group, the High Negotiations Committee, at the U.N. offices in Geneva was enough to allow him to declare the talks formally open. He previously met with a government delegation on Friday.

“We are starting officially the Geneva talks. We are actually listening with attention to the concerns of the HNC, and we are going to tomorrow discuss and listen to the concerns of the government,” de Mistura told reporters after Monday’s meeting.

De Mistura laid out the opposition’s concerns and said he planned to take up further talks in a new meeting with government representatives on Tuesday morning, before hosting the HNC again in the afternoon. He said his first goal is simply to keep the talks going, and his overall aim is to help show concrete progress for embattled Syrians.

He also tried to set a new tone, insisting these talks must be “different” from earlier ones that failed in 2014.

De Mistura also threw out a “challenge” to 17 world and regional powers – led by the United States and Russia – that held a key meeting in Vienna in November that helped pave the way for the Geneva talks: He said those powers had sent a “message” that called for “the beginning of a serious discussion about cease-fire” once the talks began.

“The challenge is now: Let’s also have those who have the capacity of discussing this at the different levels, time to discuss about cease-fire,” he said.

Minutes earlier, though, the HNC’s chief spokesman in Geneva had tough words for a key player in the process: Russia. Salem al-Mislet lashed out at Russia for its air campaign in Syria that began four months ago in support of Syrian President Bashar Assad, which has helped his troops take dozens of towns and villages in recent weeks.

“The regime is the one killing the Syrian people,” al-Mislet said when asked by a reporter working for a Russian media outlet about the participation of a representative of the militant Army of Islam group that is in the opposition’s delegation. “The regime in Russia will produce a new Hitler, and we are suffering from another Hitler in Syria.”

Earlier, the HNC said they had planned to give de Mistura a “roadmap” for implementation of their humanitarian demands on Syria that they say must happen before they formally join indirect peace talks with a government delegation in Geneva.

The day belonged to the opposition, as de Mistura’s team put off a planned morning meeting Monday to meet with the opposition – which he had not hosted yet. HNC member Farah Atassi said the opposition’s top priority was to stop the “unprecedented bombardment by the Syrian regime” of rebel-held suburbs of the capital, Damascus.

The meeting coincided with a sharp spike in violence in Syria, particularly in the rebel-held besieged town of Moadamiyeh southwest of the capital where opposition activists reported dozens of helicopter-dropped barrel bombs in the past few days.

The Geneva talks are aimed at ending a five-year conflict that has killed 250,000 people and displaced millions, leaving vast swaths of the country in ruins and fostering territorial gains of the radical Islamic State group – which is considered a terrorist group and was not invited. The talks were slow in starting, largely because of disputes over which opposition groups can take part and opposition demands that the government allow aid into besieged rebel-held areas and halt its bombardments of civilians before the talks start.

The talks are part of a process outlined in a U.N. Security Council resolution last month that envisions an 18-month timetable for a political transition in Syria, including the drafting of a new constitution and elections. That resolution was an outgrowth of the Vienna meeting in November.

In northwest Syria, advances by government forces and allied militia fighters over the weekend sent more than 3,000 people, many of them ethnic Turkmen, fleeing over the border to Turkey. The Turkish government, sees itself as the protector of Syria’s ethnic Turkmen. They are heavily concentrated in the eastern border region.

In Moadamiyeh, near Damascus, activists reported a sustained, relentless campaign of barrel bombs by the Syrian army.

The Syrian Observatory for Human Rights reported an average of around 60 barrel bombs falling on Moadamiyeh every day for the past three days. The town is located about 10 kilometers (6.2 miles) southwest of Damascus.

Ahmad Moadamani, an opposition media activist in the town, said helicopters dropped 10 barrel bombs in the span of few minutes on the southeast front of the town on Sunday evening.

He said 97 people, among them civilians, militants, and civil defense personnel, who arrived after the attack, suffered injuries.

“Most of them suffocated,” he said. He said the pattern of injuries pointed to some sort of gas attack although he acknowledged they have no way of knowing for sure what it was.

Moadamiyeh was the first major Syrian town to enter into a truce with the Syrian government in late 2014 after enduring a suffocating months-long siege during which the military pounded the community with artillery and airstrikes and refused to allow in food, medicine and fuel.

Moadamani said residents in the town had no faith in the Geneva negotiations.

“We don’t have any trust in the negotiations,” he said. “When people are dying from the hunger or at the barrel of the gun, how are we going to pursue the Geneva negotiations?”

He described the humanitarian situation in the town as “terrible in every sense of the word,” adding that conditions deteriorated after access to the town was choked off by the government on Dec. 26.

“We’ve lost most of our nutritional supplies,” he said.

Moadamani said there were more than 1,500 cases of malnutrition in the town, including 50 severe cases of malnutrition among children.

“Until today, no food or humanitarian or medical aid is allowed into the city, and no humanitarian cases are allowed to leave,” he added.

The opposition delegation said it will meet de Mistura Monday after receiving reassurances from several countries as well as the U.N. envoy regarding sieges on rebel-held areas and bombardment of civilians.

De Mistura has decided that these will be “proximity talks,” rather than face-to-face sessions, meaning that he plans to keep the delegations in separate rooms and shuttle between them. He has tamped down expectations by saying he expects the talks to last for six months.

The U.N. human rights chief, Zeid Raad al-Hussein, meanwhile told reporters in Geneva he hopes that peace talks “will lead to the end of all these horrific abuses – human rights abuses and violations of international humanitarian law – that we are all too familiar with.”

Al-Hussein added that the U.N. has a principled position that “no amnesties should be considered” for those suspected of having committed crimes against humanity or war crimes.

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Associated Press writers Jamey Keaten and Bassam Hatoum in Geneva, Omar Akour in Amman, Jordan and Philip Issa and Zeina Karam in Beirut contributed to this report.