Government forces have launched a fresh attack on the Syrian city of Homs, killing 50 people, it has been reported.
Sky News sources say 12 separate areas of the city are under siege and a Syrian National Council member said 50 people have lost their lives.
Speaking to the Huffington Post UK, 21-year-old "freedom fighter" Omar Shakir said he feared for his life, with the Syrian army firing "more than one rocket every minute" into the district of Baba Amr.
Opposition groups claim that rockets have bombarded the city, while an explosion has burst an oil pipeline leading to the main refinery.
In response, the US has reportedly closed its embassy in Damascus.
Speaking from Homs, British-born Syrian activist Danny Abdul Dayem told the BBC: "There are pieces of bodies on the street".
Dayem also told the Telegraph: "They have been hitting us with rockets for four hours now. If you don't help us they will kill millions and no-one will find out about us. Please someone help us".
One activist told Reuters: "This is the most violent bombardment in recent days".
More than 200 people were killed this weekend in the worst night of bloodshed of the 11-month uprising.
BBC journalist Paul Wood, who is inside the city, described "pretty constant shelling", adding rocket launchers were also being used. He also said there were unverified claims a field hospital had been targeted.
Homs has become the centre of armed opposition to the President Bashar al-Assad, the council said.
Speaking to Reuters, Catherine al-Talli of the Syrian National Council, said: "The tally that we have received from various activists in Homs since the shelling started at six this morning is 50, mostly civilians."
On Monday, the head of the Arab League released a statement warning that the bombing of Homs was leading leading the country towards civil war.
"We follow with great anxiety and irritation developments in the field situation in Syria, and the escalation of military operations in the city of Homs and rural areas of Damascus, and the Syrian armed forces' use of heavy weapons against civilians," it read.
Syrian State TV has blamed Monday’s violence on “terrorist groups”, who were staging the shelling and setting fire to buildings to give the impression of an attack.
Amateur video footage purporting to show a wounded man outside a hospital in Homs
The developments come after Western and Arab states reacted with outrage after Russia and China vetoed a UN resolution backing an Arab plan urging President al-Assad to give up power.
US Secretary of State Hillary Clinton called the veto a "travesty", while Number 10 said on Monday that Russia and China's veto was "incomprehensible and inexcusable."
The prime minister's official spokesperson told journalists: "Russia and China are protecting a regime which is killing thousands of people. We find their position both incomprehensible and inexcusable. By supporting that regime, they are strengthening it and allowing it to continue with that violence."
Foreign Secretary William Hague will make a statement to MPs later today amid widespread criticism of Russia and China for vetoing the latest text in New York. On Sunday, he said Russia and China would be "held responsible" for the bloodshed.
Downing Street stressed that Russia and China were alone on the 15-member Security Council in failing to vote for the resolution, which was backed by the Arab League.
Mr Cameron's spokesman said there were still ways to make progress through the UN - such as raising Syria in the General Assembly - but indicated that the UK was now concentrating on alternative courses of action, such as sanctions by the European Union.
"We will be thinking now about how we can work with other countries, both internationally and in the region, to support the Arab League plan," said the spokesman.
"The objective here is to look at how we can bring countries together and apply pressure internationally on the Assad regime.
"For example, there will be further discussion in the EU on the issue of Syria. The (EU) Foreign Affairs Council meets at the end of this month, so we are considering whether there are further measures and further sanctions.
"It is our intention to return to the UN. Clearly, we hope that Russia and China will reconsider their position. There are alternative ways of applying pressure on the Syrian regime through the UN - for example, through the General Assembly - and we are thinking about those options."
Mr Hague indicated yesterday that the UK would seriously consider severing diplomatic ties with Damascus if the Arab League followed Tunisia in expelling Syrian ambassadors. But he insisted that military action was not being considered.
Speaking on a visit to a testing facility for green homes near Watford, Deputy Prime Minister Nick Clegg said: "I'm incredibly disappointed, bitterly disappointed by the veto put in by Russia and China. I think the whole world is united in horror at what is going on in Homs. We've seen it on our television screens over the weekend.
"It is immensely frustrating that the United Nations is not being used properly to express the international community's opinion that something must change, and I think we must now work very closely with our allies in Europe and the United States to apply meaningful, consistent pressure on the regime of President Assad."
Talking to NBC's Today programme on Monday, President Obama pushed for the conflict to end without the need for military intervention, adding that a negotiated solution in Syria was "possible".
In contrast to the international community, Iran has welcomed the vetoes on the Syrian draft at the UN. Reported by The Guardian, the Iranian foreign minister told the state's official news agency: "The security council has become a tool for the west's bullying of other nations, and this time Russia and China stood up against it."
"The security council... wanted to take a path in deciding a head of state, while it is not within the functions of this council to interfere in other nations' internal affairs."
Reported by Reuters, Russian Foreign Minister Sergei Lavrov has also defended his country's veto, calling the reaction to the scuppering of the draft "hysterical". Lavrov is scheduled to travel to Damascus on Tuesday for talks with Syrian President Bashar al-Assad.