Lakhdar Brahimi, the UN-Arab League special envoy to Syria, has said that a deal that allowed some civilians to leave Homs has broken down.
Brahimi's comments came as activists said on Thursday that barrel bombs were being dropped by government forces targeting opposition-held areas in Homs after Syrian troops and pro-regime militiamen fought their way into the neighbourhoods during the course of the week, after besieging them for nearly two years.
The special envoy said on Thursday that Syria's government and opposition groups should resume talks to lift the siege on Homs.
"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.
Brahimi added that 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".
Members of the UN Security Council have also expressed "grave concern" over the plight of civilians caught in fighting in the old quarter of Homs.
"It is imperative that those who want to leave Homs are able to do so quickly and safely
Joy Ogwu, Nigeria's ambassador to the UN, who is serving as the council's current president, said the council "urged the immediate implementation" of a February resolution to improve humanitarian access in the country.
She said council members also stand behind Brahimi call to resume negotiations to end the siege.
In a statement, US ambassador to the UN Samantha Power appealed "to all those UN member states with influence on Damascus to pressure the regime to return to the negotiating table".
"It is imperative that those who want to leave Homs are able to do so quickly and safely," she said.
Syrian ambassador Bachar Jaafari told reporters that only 170 civilians were trapped in Homs, alongside thousands of terrorists - the regime's term for rebels. He said the civilians refused to leave.
Activists describe the bombardment as the worse in months.
Al Jazeera's Zeina Khodr, reporting from Antakya, in Turkey, said losing Homs would be a major blow to the opposition.
"It is not just strategic territory - lying along a major crossroad between the north and south of the country. Losing Homs would hurt the morale of the opposition," she said.
Some of the biggest anti-government demonstrations originally took place in Homs, considered the capital of the rebellion, which began more than three years ago.
Rebel-held pockets have been under a siege for nearly two years, leading to dwindling food and medical supplies.