Fresh reports of intense violence by government forces in the Syrian city of Homs have emerged as residents "begged" for international help.
The bombardment of residential areas continued on Thursday, according to the New York Times, as mortars and rockets were fired in an attempt to crush pro-democracy fighters.
Activists say the attacks have resulted in at least 200 deaths over the past week.
A video was posted online which claimed to show evidence of military planes above the city.
One resident in Homs reported to Al Jazeera English that 12 homes were targeted in the Baba Amr neighbourhood on Thursday, and that "women, men and children" had been "badly mutilated".
"We are begging the world, please do something," another activist told Al Jazeera.
Ali Hazuri, a doctor in Homs, told the AFP news agency via the BBC that a lull overnight had been broken by resumed and intense shelling
"The shells are raining down on us and regime forces are using heavy artillery," he told AFP.
The Huffington Post UK has not been able to independently verify reports from the city.
Russian foreign minister Sergei Lavrov, whose visit to President Bashar al-Assad this week was criticised for resulting in little more than empty pledges to stop the killing, previously offered to the Arab League and others.
Lavrov claimed on Thursday that the Russian diplomatic effort was "unbiased" and was more productive than the proposed UN resolution.
But activists responded that Russia had given Syria a "licence to kill".
The global protest network Avaaz launched a $1m appeal for funds to support people in Syria with medical equipment and tools for citizen journalism.
Within hours of launching, over $100,000 had been raised, the group said.
Ricken Patel, executive director of Avaaz, said: "Russia and China have given Assad a licence to kill, and the Syrian people are at the darkest part of their night.
"They need us more than ever, and citizens across the world are mobilising fast to keep Avaaz’s lifeline of support open. This could be our generation’s Spanish Civil War."
Avaaz says it has delivered more than $1.8m of medical equipment already and has smuggled almost two dozen correspondents into the country to report on the situation there.
It also said that one of its activists who had spent months in Homs photographing the violence against protesters had been killed.
Avaaz said the man known as Omar was pulled people from the rubble of a deadly massacre in Khaldiya, Homs.
His friends appealed to Avaaz and said: “We’re heartbroken, but his death will not be in vain, we will carry on the fight, but we need your support.”
British-born activist Danny Abdul Dayem, who yesterday posted video of dead children in a makeshift emergency room in Homs, said in a new video that the situation was increasingly desperate.
"All we have is about six doctors, no one can go out or get in Baba Amr, it's been surrounded by the army," he said. "There has been non-stop bombarding for four days so we are asking for your help."
Prime Minister David Cameron said Britain will help to put together a "group of nations" to promote democracy in Syria and put pressure on the regime, and pledged to deliver the "toughest possible response".
Speaking at an international gathering in Sweden, Cameron said: "It really is appalling, the scenes of destruction in Homs … we need to work with the opposition to try and help shape their future and assist them in whatever way we can.
"We also need to put together the strongest possible contact group of like-minded nations," he said. "We will continue the pressure upon this regime."
The United States, France and the Arab League have also indicated they may have to go outside the UN to build pressure on the regime, after China and Russia vetoed a resolution in the Security Council condemning the violence.
The US is consulting with individual countries about the role of the Friends of Syria grounp according to CNN.
White House spokesman Jay Carney added President Assad had missed the opportunity for dialogue.
"Rather than take that opportunity, Assad brutally cracked down on his own people. We don't think that that opportunity is available any more," he said.
UN Secretary General Ban ki-Moon joined the condemnation of Syrian president Bashar al-Assad on Thursday, and said the organisation's failure to agree on a resolution had been "disastrous" and "unacceptable to humanity".
"For too many months we have watched this crisis deepen," Secretary General Ban Ki-moon said at the UN headquarters in New York City.
"We have seen escalating violence, brutal crackdowns and tremendous suffering by the Syrian people.
"I deeply regret that the security council has been unable to speak with one clear voice to end the bloodshed."
He added that the veto on a resolution condemning the violence issued by Russia and China had been "disastrous for the people of Syria".
"It has encouraged the Syrian government to step up its war on its own people. Thousands have been killed in cold blood, shredding President Assad's claims to speak for the Syrian people."
Ban also warned that there would be "worse to come" unless the international community could take action.
"We stand ready to assist in any way that will contribute towards improvement on the ground and to the overall situation," he said.
The UN says close to 6,000 people have been killed in the country since the start of anti-government protests in March 2011.