BEIRUT — Syrian fighter jets pounded rebel areas across the country on Monday with scores of airstrikes that antiregime activists called the most widespread bombing in a single day since the Syrian uprising started 19 months ago.
The death toll for what was supposed to be a four-day cease-fire between the regime of President Bashar Assad and rebels seeking his overthrow exceeded 500. More than 80 people were killed Monday, according to observer groups.
‘‘The army is no longer able to make any progress on the ground so it is resorting to this style,’’ said activist Hisham Nijim, speaking from the northern town of Khan Sheikhoun.
Videos posted online showed jets screaming over Syrian towns, smoke rising, and residents searching damaged buildings for bodies.
A video from Maaret al-Numan in the north showed residents trying to save a boy who was buried up to his shoulders in rubble. Another showed the bodies of a boy and girl.
The strikes focused on rebel areas in the northern provinces of Aleppo and Idlib, as well as on restive areas in and around the capital, Damascus. The regime has been bombing rebel areas in the north for months, but had sparingly used its air force near the capital, presumably to avoid isolating its supporters there.
But analysts say that rampant defections and rising rebel capabilities have lessened the regime’s ability to take back and hold rebel areas, making air strikes its most effective way to fight back.
Monday was supposed to be the fourth and final day of a cease-fire to coincide with the Muslim holiday of Eid al-Adha. But violence marred the truce almost immediately after it was to go into effect on Friday.
United Nations Secretary-General Ban Ki-moon said Monday that he was ‘‘deeply disappointed’’ the warring parties did not respect the cease-fire and called on the divided international community to unite to stop the bloodshed.
‘‘As long as the international community remains at odds, the needs, attacks and suffering will only grow,’’ he told reporters in South Korea.
Envoy Lakhdar Brahimi, who represents the United Nations and the Arab League and presented the plan, told reporters in Moscow that he would keep trying to lessen the violence and ‘‘put an end to it.’’
World powers remain divided on how to stop Syria’s crisis, with the United States and many Arab and European nations calling for Assad to step down while Russia, China, and Iran continue to back the regime. But with the sides largely stalemated on the battlefield and little international appetite for military intervention, few expect the war to end soon.
Also Monday, a car bomb exploded in the Damascus suburb of Jarmana, knocking balconies off of residential buildings, according to TV news reports. The state news agency SANA said 11 people were killed and 67 wounded. SANA also reported a second car bomb in the area later Monday but did not give a number of dead and wounded.
The Britain-based Syrian Observatory for Human Rights, which compiles daily death tolls, said more than 500 people were killed during the four-day holiday. It said more than 80 people were killed Monday and that the number was expected to rise.
Before the holiday, about 150 people were reported dead each day. Antiregime activists say more than 35,000 people have been killed since the uprising started in March 2011.
Rami Abdul-Rahman, head of the Observatory, said the number of airstrikes spiked Monday. ‘‘Today has seen the most intense air raids across Syria since the start of the uprising,’’ he said.
Muhieddine Lathkani, a London-based member of the Syrian National Council opposition group, said the air attacks showed the regime’s ‘‘total despair’’ and the military’s inability to recapture rebel areas.
Among the hardest hit areas was the northern town of Maaret al-Numan, which rebels seized earlier this month only to face heavy retaliation.
Activist videos showed fighter jets dropping bombs over Damascus suburbs including Yabroud, Hazza, and Harasta. Other videos from the poor neighborhood of Hajar al-Aswad in south Damascus showed what activists said were people killed by shelling. One video showed a dead family of five, all wrapped in blankets.
The videos could not be independently verified but correspond to other AP reporting on the events.
Abdelbaset Sieda, president of the Syrian National Council, said the Syrian regime was losing its grip on power and that the opposition must be prepared to rebuild.
The Syrian opposition has failed to come up with a united leadership and has been riddled by disagreements over the role of Islam in the uprising and the best way to topple Assad.