Syrian warplanes heavily bombarded rebel targets in the suburbs of Damascus on Monday in what activists said was one of the most intense air raid campaigns around the capital since the uprising began 19 months ago.

A government official said a car bomb killed 10 people on the outskirts of the city. TV footage showed firemen fighting the blaze amid wide destruction after parts of balconies fell on cars parked on a residential street. As smoke billowed, a woman was seen running away with children from the area of the blast and electricity cables dangled from poles. Activists said the air raids were launched both before and after the car bomb and were still under way.

At least 150 killed Sunday, activists say

Monday was supposed to be the fourth and final day of a UN-backed ceasefire to coincide with the Muslim holiday of Eid al-Adha, one of the holiest periods on the Muslim calendar. But the truce was violated almost as soon as it was supposed to take effect on Friday and violence continued unabated over the holiday weekend. Activists said at least 150 people were killed Sunday, a toll similar to previous daily casualty tolls.

The army warned late Sunday night that it will strike "remnants of terrorists with an iron fist" after they "repeatedly violated the ceasefire." The regime of President Bashar al-Assad often refers to those waging the uprising as "terrorists."

Mohammed Saeed, an activist based in the Damascus suburb of Douma, said there were at least 15 air raids on the suburbs early Monday. The Britain-based activist group Syrian Observatory for Human Rights said warplanes carried out at least six air raids on Damascus suburbs including Rankous and Harasta. It said there are intense clashes in those areas as troops try to regain control from the rebels.

"Members of the Free Syrian Army are shooting at the planes without succeeding in shooting them down," said Saeed referring to the main rebel group fighting Assad's forces. "The air raids today are unusual in their intensity, although we have seen worse days."

'This crisis cannot be solved with more weapons and bloodshed... As long as the international community remains at odds, the needs, attacks and suffering will only grow.'—UN Secretary General Ban Ki-moon

A Syrian official said the car bomb in the Damascus suburb of Jaramana also wounded 41 people and heavily damaged shops and apartments in the area heavily inhabited by Christians and members of the Druse minority sect.

The Observatory also reported clashes and shelling in other parts of the country including the northwestern province of Idlib that borders Turkey, where it said warplanes carried out 11 air raids on several villages. Amateur videos showed warplanes in the skies, then giant mushroom clouds of smoke after the missiles hit.

On Friday, at least 15 people were killed in a Damascus car bomb, state media said.

UN Secretary General deeply disappointed

UN Secretary General Ban Ki-moon expressed deep disappointment at the collapse of the ceasefire and urged more unity from the international community. Speaking in South Korea, he said the UN is trying to ease Syria's humanitarian woes and find a political solution to the crisis.

UN Secretary General Ban Ki-moon delivers a speech about the collapse of the UN-backed ceasefire at Monday's award ceremony for the Seoul Peace Prize in Seoul, South Korea. (Ahn Young-joon/Associated Press)

He called for an immediate halt to the fighting and said other countries and the United Nations need to do more to help.

"I am deeply disappointed that the parties failed to respect the call to suspend fighting. This crisis cannot be solved with more weapons and bloodshed," he said. "I remain committed to doing all I can to make this happen. As long as the international community remains at odds, the needs, attacks and suffering will only grow."

In Turkey, state-run Anadolu news agency said the Turkish forces fired artillery in response to a stray shell fired from Syria that landed across the northern border. The shell landed some 300 metres away from the Turkish border village of Besaslan. No one was injured, but a power line was destroyed.

With the unraveling of the ceasefire, it's unclear what the international community can do next. The holiday truce marked the first attempt in six months to reduce the bloodshed in Syria, where activists say more than 35,000 people have been killed in 19 months.

Syrian opposition plans for post-Assad future

In Turkey, about 150 members of the Syrian opposition met Monday to plan for a post-Assad future, discussing the immediate challenges of managing parts of the northern Idlib province, sections of the city of Aleppo, the country's largest, and other areas that are held by rebels. Long-term planning will focus on constitutional and legal reform, laws on elections and political parties, and how to build a modern national army.

Delegates to the three-day meeting at a hotel on the outskirts of Istanbul included members of Syrian rebel groups as well as the country's Kurdish minority. Abdelbaset Sieda, president of the Syrian National Council, said the Syrian regime, which he described as a "criminal group," was losing its grip on power and that the opposition must be prepared to rebuild the devastated country.

"The transitional phase has started now," Sieda said. "That's what we're witnessing clearly today in many of our cities and villages."