Twin suicide bombings killed at least 55 people and wounded nearly 400 in the Syrian capital Damascus on Thursday, authorities said, in the deadliest attacks of the country's 14-month uprising.
The government and the opposition traded blame, with Syria's foreign ministry, in a letter to UN chief Ban Ki-moon hours after the attacks, saying they were the work of "terrorists" armed and funded by foreign organisations and media.
The blasts during morning rush hour left an apocalyptic scene of destruction and further put into question a UN-backed ceasefire that has failed to take hold since it went into effect on April 12.
Ban strongly condemned Thursday's attacks and urged all sides to "distance themselves from indiscriminate bombings and other terrorist acts," his spokesman Martin Nesirky said.
Washington called the attacks "reprehensible" while UN-Arab League envoy Kofi Annan, who brokered the truce, described them as "abhorrent".
Russia and China, both supporters of President Bashar al-Assad's embattled regime, called for a stop to the violence and urged all parties in Syria to cooperate with Annan's peace plan.
State television aired gruesome footage of the aftermath of the twin explosions in the neighbourhood of Qazzaz, also blaming "terrorists", a term used by authorities to refer to rebels seeking to topple Assad's regime.
The television showed images of a woman's charred hand on a steering wheel, her gold bracelets dangling from her blackened wrist.
Other burnt and mangled bodies lay in the street amid the carcasses of smouldering vehicles and rubble.
"Is that the freedom you want? Students from schools and employees going to work are dead," shouted one man in the middle of the destruction.
The explosions took place on a main freeway in the south of Damascus, in front of a nine-storey security complex whose facade was heavily damaged while nearby residential buildings collapsed.
The interior ministry said the suicide attackers used a tonne of explosives, killing at least 55 people and wounding 372.
It added that emergency workers filled 15 bags with body parts, and that the blasts also destroyed around 200 cars.
"These crimes show that Syria is targeted by a terrorist attack launched by organisations armed and funded by parties who proclaim their backing to terrorist crimes," state news agency SANA quoted the foreign ministry as saying.
At the United Nations, Syria's ambassador said that recent bomb attacks in Syria "carried the stamp of Al-Qaeda methods," adding that British, French and Belgian nationals were among foreign fighters killed in recent clashes.
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But the opposition Syrian National Council accused Assad's regime of staging the bombings in a bid to undermine the UN observer mission and to persuade the international community that Damascus was battling "terrorists."
"This is the only way for the regime to claim that what is happening in Syria is the work of terrorist gangs and that Al-Qaeda is expanding its presence in Syria," said Samir Nashar, of the exile group's executive branch.
The SNC accused the regime of placing the bodies of people it had killed at the site of the bombings, to claim that they died in the blasts.
"Among the victims of the attacks are those whose names are on the lists of people imprisoned by the regime," the group said in a statement.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the Damascus bombings targeted an intelligence base and killed 59 people, including civilians and security personnel.
The attacks came a day after UN observers monitoring the ceasefire escaped unharmed when a roadside bomb exploded as they were visiting the flashpoint southern city of Daraa. Ten Syrian troops escorting them were hurt.
In Geneva, Annan said through his spokesman Ahmad Fawzi that he "condemns in the strongest possible terms the attacks that took place earlier today in Damascus."
"These abhorrent acts are unacceptable and the violence in Syria must stop," he added.
In Washington, State Department spokeswoman Victoria Nuland said in a statement: "Any and all violence that results in the indiscriminate killing and injury of civilians is reprehensible and cannot be justified."
Damascus has been the target of a number of bombs in the past few months.
Suicide bombers hit two security service bases in the capital on December 23, killing 44 people, in attacks the regime blamed on Al-Qaeda but which the opposition said were the work of the regime itself.
The UN leader had warned on Wednesday of a "brief window" to avoid civil war and indicated the future of the ceasefire monitoring mission was in doubt.
Highlighting an "alarming upsurge" of roadside bombs, alongside government attacks, Ban said that both sides "must realise that we have a brief window to stop the violence, a brief opportunity to create an opening for political engagement between the government and those seeking change."
If the violence did not stop, Ban said he feared "a full-scale civil war with catastrophic effects within Syria and across the region."
Elsewhere in the country on Thursday, at least 14 people died in violence, including a child killed by army shelling in northwestern Idlib province, the Observatory said.
The watchdog says that more than 12,000 people, mostly civilians, have been killed in Syria since the revolt broke out in March last year.