William Hague condemned suicide bombings that killed at least 55 and injured 300 in the Syrian capital Damascus today and urged the regime to implement a full ceasefire.
The Foreign Secretary said civilians continued to pay the price for its failure to end repression and violence despite agreeing to a United Nations peace plan.
In the deadliest such terror attacks since the uprising against President Bashar Assad's government 14 months ago, two explosions tore the front off a military intelligence building.
The government and opposition blamed each other for the bloodshed but there were growing concerns that it was a sign that al Qaida-inspired terror groups were beginning to exploit the chaos.
Continued violence has pushed the ceasefire plan negotiated by UN envoy Kofi Annan - which is being overseen by a team of monitors - to the brink of collapse.
Expressing his condemnation, Mr Hague said: "Yet again it is the people of Syria who are suffering as a result of the repression and violence, which must come to an end.
"As Kofi Annan made clear to the UN Security Council earlier this week, the onus is on the Syrian authorities to implement a full ceasefire and begin the political dialogue required by the Annan Plan. The UK supports fully the work of the UN and the Joint UN/Arab League Special Envoy and their efforts to end the violence and resolve the crisis in Syria.
"To that end it remains critical for the UN mission to deploy in full, and for monitors to cover as many locations as possible, as soon as possible."
He paid tribute to the UN monitors, saying an attack on one of their convoys in Deraa yesterday "shows how difficult and dangerous conditions are on the ground".
"I urge the Syrian government to implement rapidly and fully their commitments under the six point plan. And I call on the opposition to take all steps necessary to bring about a sustainable end to the violence, he concluded.