Two powerful blasts in quick succession have killed dozens of people and injured more than 300 others in the Syrian capital Damascus, according to Syrian officials and state media.
One of Thursday's rush-hour explosions hit the Qazaz neighbourhood that houses a well-known military intelligence headquarters involved in President Bashar al-Assad's crackdown on a 14-month uprising.
A much larger blast followed, shaking the same neighbourhood.
The interior ministry said at least 55 people were killed and 372 wounded in the blasts.
"Two explosions caused by terrorists took place on the road in the south of Damascus," state TV reported, adding that the blasts occurred "as people were heading to work and children to school".
Syria's foreign ministry said the attacks were a sign that the country was facing foreign-backed terrorism and urged the UN Security Council to combat countries or groups supporting such violence.
"Syria stresses the importance of the UNSC taking measures against countries, groups and news agencies that are practicing and encouraging terrorism," the state news agency SANA quoted the ministry as saying in a letter to the UN body.
Bashar Jaafri, Syrian ambassador to the UN, accused some Arab countries of supporting attacks on his country.
Third party suspected
Both the government and the opposition blamed each other for the deadly explosions, but a retired Lebanese army major general, Hisham Jaber, told Al Jazeera a third group may be involved.
"I am not accusing al-Qaeda, but everybody knows there are groups from al-Qaeda or similar or belong to it or they have the same objective and belief," he said.
In-depth coverage of escalating violence across Syria
"We do estimate the number of those groups, who came from Libya, Yemen, north Lebanon and Iraq, at about 800-1000. These are well-indoctrinated and well trained."
Ban Ki-moon, UN secretary general, has urged both the government and the opposition to distance themselves from such attacks.
“The secretary-general reiterates his urgent call on all sides fully to comply with their obligations to cease armed violence in all its forms,” a statement issued by Ban's office said.
The US condemned the attacks as "reprehensible" while UN-Arab League envoy Kofi Annan, who brokered the truce, described them as "abhorrent".
The EU also denounced the bombings as "pure terrorism", but said Annan's peace plan was still viable.
Russia and China, both supporters of Assad's government, called for a stop to the violence and urged all parties in Syria to co-operate with Annan's peace plan.
Heavily damaged cars
An Associated Press reporter at the scene said medical workers were collecting human remains from the streets and that heavily damaged cars and pickup vehicles stood smoldering in the area after the explosions.
The outer wall of the nine-storey headquarters collapsed, although the structure inside appeared intact.
Al Jazeera's Rula Amin, reporting from Beirut in neighbouring Lebanon, said the explosions had been heard all over the city.
"Damascus residents from different parts of town all heard the explosions. People were really shaken by the power of these blasts. These attacks - every day - become more efficient and more deadly."
Syrian TV showed dozens of mangled, burnt and smoldering vehicles, some containing incinerated human remains.
A large crater could be seen in the road and at least one lorry had been overturned.
One resident, who said he got within 100 metres of one of the blasts before being turned back by security forces, told Reuters news agency he saw broken glass on the street and women crying. Nearby schools had sent children home for the day.
Major-General Robert Mood, chief of a UN observer mission in Syria, rushed to the site of the blasts shortly after they took place to survey the damage, an AFP correspondent reported.
"This is yet another example of the suffering brought upon the people of Syria from acts of violence," he said as he visited the site of one of the blasts.
"We, the world community, are here with the Syrian people and I call on everyone within and outside Syria to help stop this violence."
The attacks came a day after Mood escaped unharmed when a roadside bomb exploded as he led a team of UN observers into the southern flash point city of Deraa.
The Syrian Observatory for Human Rights, a London-based rights group, has said that almost 12,000 people, mostly civilians, have been killed in Syria since the revolt, inspired by Arab Spring uprisings, broke out in March last year.
About 800 of them have died since a UN-backed truce was supposed to have taken effect on April 12.
Damascus was hit by two blasts on May 6, with three soldiers wounded in one of the attacks. Television footage showed a mangled car destroyed by one of the explosions.
A deadly suicide bombing at Zein al-Abidin mosque in Damascus' central Midan district on April 27 killed 11 people and wounded dozens, according to state media.
An armed group calling itself Al-Nusra Front had earlier claimed responsibility for a deadly suicide bombing on April 20 near the Syrian city of Hama that targeted a restaurant used by the security forces.
The Syrian authorities regularly blame the blasts on what it calls "terrorist groups".