Twin suicide car-bomb attacks have killed at least 55 people and wounded 372 in Damascus, Syrian officials say. The blasts happened near a military intelligence building during morning rush hour. State TV showed burnt cars and two deep craters in the road. The government and the opposition blamed each other for the attack - the deadliest in Damascus since the start of Syria's uprising 14 months ago. The two sides are supposed to observe a ceasefire monitored by a UN team. However, violence has continued unabated across the country, with the restive city of Homs shelled again overnight. International peace envoy Kofi Annan - who brokered a six-point peace plan - said the Damascus blasts were "abhorrent" and counter-productive. The US also condemned them. A statement by the state department said indiscriminate killing could not be justified, and urged the Syrian government to "fully and immediately implement the Annan plan" to prevent further escalation. The call was echoed by UK Foreign Secretary William Hague, who said the people of Syria were "suffering as a result of the repression and violence, which must come to an end". Body parts Central Damascus is under the firm control of government forces, but the city has been hit by several bombings in recent months, often targeting security buildings or military convoys. Thursday's attacks occurred in the southern suburb of al-Qazzaz shortly before 08:00 (05:00 GMT), as people were going to work. At one of the main Damascus hospitals, a doctor says it is the worst emergency he has seen, the injuries are severe. We see more than body bags at the morgue - the corpses are charred sticks, with limbs torn off. Whoever did this wanted to cause the most damage possible because the blast took place during early morning rush hour. Every time something happens in Syria, there are two narratives. We have spoken to angry people who blame Saudi Arabia, Qatar and Turkey - countries which have come out in support of the Syrian opposition. Grieving families cursed the opposition. The opposition alleges as always that this was the cynical work of the government itself to try to discredit the opposition. The interior ministry said "foreign-backed terrorists" carried out the attacks, using two cars "loaded with more than 1,000kg of explosives and driven by suicide bombers". The explosions damaged the facade of a 10-storey military intelligence building involved in the crackdown on the uprising against President Bashar al-Assad's rule. It is part of a broader military compound of the Palestine Branch, one of the most feared among the more than 20 secret police organisations in the country, correspondents say. The unit was originally set up in the 1950s to interrogate suspected Israeli spies. But over the past decade, it has evolved into the country's counter-terrorism unit, and is infamous for interrogations and torture, they say. Large crowds gathered despite the security cordon, shouting slogans and chants in support of President Assad. All the while, the emergency teams looked for body parts and bulldozers were at work clearing away some of the debris. The whole area looks like a wasteland, says the BBC's Lyse Doucet at the scene. One Damascus resident, who gave his name as Ahmad, told the BBC the blasts were the biggest explosions he had ever heard. "The whole of Damascus heard them. At first, I thought they were air strikes," he said. The Norwegian head of the UN observer mission in Syria, Maj Gen Robert Mood, visited the site. He said the Syrian people did not deserve this "terrible violence". Homs shelling The opposition says the attacks were carried out by the government itself to discredit rebel forces. Meanwhile, the Syrian army bombarded the city of Homs overnight. Opposition activists described the bombardment as some of the heaviest shelling in Homs for weeks. Eleven UN monitors are now stationed there to try to implement a ceasefire. The observers are in the country as part of the joint UN-Arab League peace plan and began deploying last month. There are now 105 monitors in the whole country, but their presence has had no effect in quelling the violence. A total of 300 are due to be sent in the coming weeks. The UN says at least 9,000 people have died since pro-democracy protests began in March 2011. In February, Syria's government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.