Syrian opposition representatives Tuesday said only "clear and immediate action" by the international community could save peace talks in Geneva, as government troops backed by intense Russian airstrikes threatened to cut off key rebel supply lines to the northern city of Aleppo.
The delegation postponed a meeting with UN envoy Staffan de Mistura amid deepening concern in opposition circles after regime forces captured a string of villages just east of the last rebel lifeline extending from Aleppo north to the Turkish border.
The Syrian Observatory for Human Rights said that Russian warplanes backing up President Bashar al-Assad's forces have carried out some 320 airstrikes since Monday.
Syrian ground troops and militiamen took the villages of Tel Jebeen, Hardatneen and surrounding areas, while at least 18 civilians were killed in the airstrikes, Observatory director Rami Abdel-Rahman said.
The advance took the government forces within 5 kilometres of linking up with the besieged pro-government villages of Nubul and Zahraa and cutting the road to the border, the Observatory said.
"The pro-regime offensive on [northern] Aleppo is no coincidence - it's cynically timed military realpolitik; they seek rebel withdrawal from talks," analyst Charles Lister of the Washington-based Middle East Institute wrote on Twitter.
The opposition's Higher Negotiations Committee, meeting in Geneva, condemned the civilian deaths and said that they called for "clear and immediate action by the international community."
"Only through such action will it be possible to continue the political process," the Committee warned, adding that its decisions "will depend on what happens in that regard."
The eastern half of Aleppo, contested between regime and rebel forces for almost four years, is the last major urban centre in rebel hands.
Cutting it off from the northern route to Turkey and lifting the siege of Nubul and Zahraa would be a major victory for the government.
Recent weeks have seen rebels, hit hard by four months of the Russian air campaign, lose ground on key front lines north and south of Aleppo, in the north-western mountains and in the far south of Syria.
Russia's Defence Ministry on Monday said that its warplanes had carried out over 1,350 strikes on "terrorist targets" in Syria over the previous week, state news agency TASS reported.
The developments came after de Mistura held his second meeting with government delegates, where he was expected to discuss opposition demands for concrete humanitarian measures ahead of any substantive political talks.
The opposition, in their first formal meeting with de Mistura on Monday night, called for a lifting of all sieges imposed by regime forces, the release of detainees and an end to Syrian and Russian airstrikes.
The chief government negotiator, Syrian Ambassador to the United Nations Bashar al-Jaafari, said that the talks were in a preparatory phase.
"We still don't know names of the other side's delegation or the agenda," he told media after the meeting.
Al-Jaafari hit back at the opposition stance, saying the government side was committed to talks without preconditions and accusing the opposition delegates of acting "like amateurs and not professional politicians."
World powers hope that the indirect talks will help bring about a ceasefire in the five-year long conflict, which has cost some 250,000 lives and driven over 11 million people from their homes, according to UN estimates.
However, the opposition is insisting on immediate humanitarian measures while the government says combatting terrorism must be the first priority.
Meanwhile, talks on humanitarian aid jointly hosted by Britain, Germany, Kuwait, Norway and the UN were set for Thursday in London.
The conference was to seek pledges to meet the UN aid appeal estimated at 7.7 billion dollars.
US President Barack Obama told British Prime Minister David Cameron that the United States will make "significant new contributions" to humanitarian aid for Syria, but the White House did not provide figures.
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