Twin suicide car bombs exploded outside a military intelligence building and killed 55 people Thursday, tossing mangled bodies in the street in the deadliest attack against a regime target since the Syrian uprising began 14 months ago.

The bombings fuelled fears of a rising Islamic militant element among the forces seeking to oust President Bashar al-Assad and dealt a further blow to international efforts to end the bloodshed.

The first car bomb went off on a key six-lane highway during the morning rush hour, knocking down a security wall outside the government building and drawing people to the scene, witnesses said.

A much larger blast soon followed, shaking the neighbourhood, setting dozens of cars ablaze and sending up a grey mushroom cloud visible around the capital.

Syrians inspect the site of twin blasts in Damascus on Thursday. (Louai Beshara/AFP/Getty Images)

Syrian state TV video showed dozens of bodies, some charred or dismembered, strewn in the rubble or still inside damaged cars. An Associated Press reporter at the scene saw medics in rubber gloves picking through the site for human remains amid the two craters that were blasted into the asphalt.

The Interior Ministry, which oversees the police and security services, said 55 people were killed and more than 370 were wounded.

Officials said suicide bombers detonated explosives weighing more than 1,000 kilograms.

"The house shook like it was an earthquake," Maha Hijazi said, standing outside her home nearby.

World powers seeking to halt Syria's unrest condemned the attack and urged all sides to adhere to a ceasefire brokered by UN and Arab League envoy Kofi Annan.

"In order to prevent another escalation of violence, we continue to call on the Syrian regime to fully and immediately implement the Annan plan," State Department spokeswoman Victoria Nuland said in Washington.

British Foreign Secretary William Hague said "the onus is on the Syrian authorities to implement a full ceasefire and begin the political dialogue required by the Annan plan," while the UN Security Council said in a statement that "any acts of terrorism are criminal and unjustifiable regardless of their motivation."

'It is only going to create more suffering'

Maj. Gen. Robert Mood, the Norwegian head of a team of observers overseeing the ceasefire, toured the site and said the Syrian people do not deserve this "terrible violence."

"It is not going to solve any problems," he said. "It is only going to create more suffering for women and children."

Two Syrian soldiers, left, and civilian citizens carry a dead body after the two explosions. (SANA/Associated Press)

Annan, too, appealed for calm.

"The Syrian people have already suffered too much," he said in a statement.

The blast was the largest and most deadly yet in a series of bombings targeting state security buildings since last December.

Most of these have been in Aleppo and Damascus, Syria's two largest cities, which have generally stood by Assad since the popular uprising against his rule broke out in March 2011.

Family narrow escapes bomb blasts

The CBC's Laura Lynch visited the site of the explosions Thursday, and said the extent of the damage was "just incredible."

"There were two huge craters where the car bombs had actually exploded right next to a military intelligence building, and the facade had been ripped off of that building," Lynch said by phone.

"But the radius of damage was much bigger than that. It went all the way across quite a wide street and was hitting homes that were sitting across the street."

Lynch spoke to one resident in the area who woke up as the first explosion erupted. The second blast punched a large hole through his bedroom, but he and his family narrowly escaped uninjured.

"I am amazed that he and his wife and his three daughters survived because their bedrooms face onto the street, and they were all in bed when the first bomb hit," Lynch reported. "He went and grabbed his girls and he pushed them back into the apartment just before the second bomb hit, and their bedrooms are just a wasteland."

Regime blames foreign terrorists

The government blamed the attack on armed terrorists it says are driving the uprising, which has grown into the strongest threat to the Assad family dynasty in its four decades in power.

The Syrian Foreign Ministry sent letters to UN Secretary-General Ban Ki-moon and the head of the Security Council, asking the body "to take steps against states, parties and media outlets that practice and encourage terrorism," Syria's state news service said.

Syria's UN ambassador, Bashar Ja'afari, told the Security Council that a second bombing in Aleppo on Thursday also killed civilians and damaged property.

The Britain-based Syrian Observatory for Human Rights said five intelligence officers were killed when a bomb targeted their car in Aleppo. It was unclear if this was the same event.

A leader of the Free Syrian Army, an umbrella group of anti-regime militias throughout the country, condemned the Damascus attack and denied the group was involved. Capt. Ammar al-Wawi accused the government of staging the attack to turn the world against the uprising.

There was no immediate claim of responsibility, but a shadowy militant group calling itself the Al-Nusra Front has claimed past attacks through statements on militant websites. Little is known about the group, although Western intelligence officials say it could be a front for al-Qaeda's Iraq branch.

With files from CBC News