William Hague accused the Syrian regime of "cold-blooded cynicism" as the reported deaths of 200 people in one of the bloodiest attacks of the uprising fuelled calls for United Nations action.
The Foreign Secretary spoke out after the Syrian army and security forces launched a sustained barrage of mortars and artillery against the city of Homs overnight - and appealed for international unity.
A Security Council vote on draft resolution supporting an Arab-led call for President Bashar Assad to step down is scheduled for this afternoon but has faced resistance from Russia.
Mr Hague said it was time for countries to stop giving "shelter" to the regime after the assault on Homs - which has been at the centre of 11 months of protests.
"The Syrian regime's actions display President Assad's cold-blooded cynicism in the face of mounting international pressure for the UN Security Council to do its utmost to end the bloodshed.
"The escalating violence underlines the critical importance of the UN Security Council adding its weight to the Arab League's efforts to end the crisis in Syria," he said.
"The time is long past for the international community, particularly those that have so far sheltered the Assad regime, to intensify the pressure to end over 10 months of violence."
Mr Hague said it was "all the more chilling" that the latest casualties were sustained on the 30th anniversary of the Hama massacre.
Around 20,000 people died in the 1982 operation by the Syrian army - then under the command of president Assad's father.
"I condemn unequivocally the use of tanks, mortars and artillery in civilian areas," Mr Hague said.
There were six arrests this morning amid protests at the Syrian embassy in London.