David Cameron has warned Syrians that if President Bashar Assad remains in power it makes the risk of "all-out civil war" more likely.
The Prime Minister said the UK would make a fresh push for a United Nations Security Council resolution to demand an end to violence and for humanitarian access to the Baba Amr district of Homs, scene of some of the bloodiest violence.
"The history of Homs is being written in the blood of its citizens," he told MPs.
Mr Cameron said he had spoken to British photographer Paul Conroy, who was injured in an attack which killed Sunday Times journalist Marie Colvin in Homs.
In a statement to the Commons, Mr Cameron said: "He described vividly the barbarity he had witnessed in that city."
Labour leader Ed Miliband said: "The pictures and testimony coming out of Homs in the last few days and again today are truly horrific, with women and fathers telling of their children being murdered in front of their eyes."
Mr Cameron, updating MPs on last week's European Council, said the UK was playing a leading role in attempts to forge an international coalition to put pressure on Assad and alleviate the suffering of Syrians.
He said Britain has provided an extra £2 million to help deliver emergency medical supplies and basic food rations for over 20,000 people, but there were problems in getting the aid to those who needed it.
"Now that the Syrian government has occupied Baba Amr, it has a duty to allow humanitarian access to alleviate the suffering that it has caused," he said.
"Britain will be working this week to secure a United Nations Security Council resolution which demands an end to the violence and immediate humanitarian access. The longer access is denied, the more the world will believe that the Syrian regime is determined to cover up the extent of the horror that it has brought to bear on Baba Amr."