Syria's main opposition bloc has given warning that attacks by government forces in Aleppo province could stand in the way of the peace talks under way in the Swiss city of Geneva.
The Syrian government launched a major offensive from the north of Aleppo and captured several strategically important towns on Monday.
The warning was sounded by the Higher Negotiations Committee (HNC), which sent a 17-strong opposition team, including three rebel leaders, to Geneva on Saturday.
Salim al-Muslet, HNC spokesman, said the opposition was waiting for reaction to the developments in Aleppo and other provinces.
"It is important for us to see the lifting of sieges of children starving to death," he said.
"Since last night, big massacres have taken place in Syria and nobody is doing or saying anything.
"We do not know if the international community is completely blind or they do not want to do anything. We are here to know if they are keen to do anything."
The HNC condemned the Aleppo offensive, saying it showed President Bashar al-Assad's government is not committed to finding a peaceful resolution to the conflict.
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The UK-based Syrian Observatory for Human Rights confirmed on Tuesday the reports from Aleppo, saying that at least 270 air strikes had been documented in the last 24 hours.
The monitoring group said at least 11 civilians, including two Red Crescent volunteers, were killed in the attacks - in addition to 39 government troops and rebel fighters.
"The sense of cynicism here among the Syrian opposition is just as high as it is among the regime," Al Jazeera's Mohamed Jamjoom, reporting from Geneva, said.
The meetings are part of a process outlined in a UN resolution last month that envisages an 18-month timetable for a political transition, including the drafting of a new constitution and elections.
Our correspondent said the HNC met earlier on Tuesday for two to three hours before deciding to postpone their meeting with Staffan de Mistura, the UN envoy.
"The opposition continues to maintain that these talks cannot actually begin in good faith until the Syrian regime actually lifts the siege and allows the entry of humanitarian aid and the release of political prisoners as a sign of good faith," he said.
Monday's military advance in Aleppo opens the way for Assad's forces to retake Syria's largest city as troops marched through the towns of Hardatnein, Tal Jibbeen and Deir Zaitoun.
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Hundreds of families were reportedly fleeing their homes because of heavy Russian air strikes assisting the Syrian government's advance.
Separately, John Kerry, US secretary of state, has also given warning that "the situation on the ground for the Syrian people is unfathomable".
"We haven't seen a catastrophe like this since World War II, and it's unfolding before our eyes," he said at a conference in Rome on Tuesday on countering the advances of the Islamic State of Iraq and the Levant (ISIL) group.
The Geneva peace talks are the first since two rounds of negotiations collapsed in 2014.
"When the weak negotiates with the strong, it is the strong that dictates. What is going on in Geneva is deformed on so many levels," Marwan Bishara, Al Jazeera's senior political analyst, said.
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"These are asymmetrical talks - talks between the opposition that is being bombarded every day and a regime that is responsible for the disaster."
The Geneva negotiations are meant to develop a "road map" to end the nearly five-year conflict that has resulted in more than 250,000 Syrians being killed.
The conflict has also displaced millions more and sent hundreds of thousands fleeing as refugees to Europe.
|Opposition delegates said in Geneva on Tuesday "big massacres have taken place and nobody is doing or saying anything" [EPA]
Source: Al Jazeera