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Syrian warplanes launched 60 airstrikes against rebel targets around the country on Monday, the most intense air raids across the country since the uprising began 19 months ago, according to anti-regime activists. The suburbs of the capital Damascus were particularly hard hit.

Activists said at least 500 people were killed over the four-day period ending Monday when a U.N.-backed truce was supposed to be in effect. They said the death toll for Monday so far has reached 80 and would likely rise further. In the period leading up to the truce, there was an average of about 150 deaths per day in the civil war, according to activists.

A government official said a car bomb killed 10 people on the outskirts of Damascus and 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.

Another car bomb exploded in a Damascus neighborhood where rebels are active, and state-run news agency said there were many casualties.

Monday was supposed to be the fourth and final day of a U.N.-backed cease-fire 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.

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

Rami Abdul-Rahman, who heads the Britain-based Syrian Observatory for Human Rights, said airstrikes on Monday were by far higher than on any other day since the conflict began in March last year.

"Today has seen the most intense air raids across Syria since the start of the uprising," he said, estimating there were more than 60 airstrikes nationwide by early afternoon Monday.

He said the Syrian military was trying to compensate for recent losses on the ground with airstrikes.

Muhieddine Lathkani, a London-based member of the Syrian National Council opposition group, said the air attacks were a result of the regime's "total despair" and reflect the military's inability to recapture areas it lost to the rebels.

Mohammed Saeed, an activist based in the Damascus suburb of Douma, said members of the rebel Free Syrian Army were shooting at the planes but failing to bring them down.

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.

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

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."

U.N. International peace envoy Lakhdar Brahimi told reporters in Moscow that the failure of the cease-fire will not discourage him and his supporters.

"So we will continue to work as hard as we possibly can, in cooperation with everybody inside of Syria and outside of Syria to bring the level of violence, put an end to it," he said.

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 meters (yards) away from the Turkish border village of Besaslan. No one was injured, but a power line was destroyed.

With the unraveling of the cease-fire, 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.

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."