Syria's government and opposition groups should resume talks to lift the siege on Homs, where bombardments have re-started, international mediator Lakhdar Brahimi said on Thursday.
UN-Arab League envoy Brahimi said the discussions had been well underway between the Syrian government and "a negotiating committee representing the civilians and fighters still trapped in the Old city of Homs as well as the inhabitants of the Al-Waer neighborhood."
"It is a matter of deep regret that negotiations were brutally stopped and violence is now rife again when a comprehensive agreement seemed close at hand," Brahimi added in the statement distributed at the UN headquarters in New York.
"We urge all the parties to return to the negotiating table and complete the deal which was on the verge of being signed," he said.
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"We have reached out to all those who could help put an end to this tragedy."
The Syrian army launched an assault Tuesday against the rebel-held neighborhoods in Homs and "have achieved key successes" and "killed a number of terrorists," Syrian television said, using the regime's term for rebels.
Homs is Syria's third city, and activists have long referred to it as the "capital of the revolution" because of the huge pro-democracy protests held there when the uprising began in March 2011.
Most of the central city is now under regime control. Rebel-held pockets have been under a siege for nearly two years, leading to dwindling food and medical supplies.
According to rebel groups, around 1,300 people, mainly combatants, are still trapped inside army-besieged neighborhoods, after around 1,400 civilians were evacuated at the beginning of the year.