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

Activists said at least 500 people were killed over the four-day period ending yesterday, when a UN-backed truce was supposed to be in effect. They said Monday's death toll so far had reached 80 and was likely to rise. In the period leading up to the truce, deaths averaged 150 a 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 began before and after the car bomb and were continuing.

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

Yesterday was supposed to be the fourth and final day of the 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.

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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 Monday's airstrikes were by far higher than on any other day since the conflict began.

"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.

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.