Syrian fighter jets pounded rebel areas across the country on Monday with scores of airstrikes that anti-regime activists called the most widespread bombing in a single day since Syria's troubles 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, and activists guessed the government's heavy reliance on air power reflected its inability to roll back rebel gains.

"The army is no longer able to make any progress on the ground so it is resorting to this style," said activist Hisham Nijim via Skype from the northern town of Khan Sheikhoun.

Activists said more than 80 people were killed nationwide Monday while videos posted online showed fighter jets screaming over Syrian towns, mushroom clouds rising from neighborhoods and residents searching the remains of damaged and collapsed buildings for bodies. One 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 dead bodies of a young boy and girl laid out on a tile floor.

The airstrikes 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 an internationally sanctioned cease-fire to coincide with the Muslim holiday of Eid al-Adha, one of the holiest periods of the Muslim calendar. But violence marred the truce almost immediately after it was to go into effect on Friday and continued apace through the weekend.

U.N. Secretary-General Ban Ki-moon said Monday he was "deeply disappointed" that the warring parties didn't 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 U.N. and the Arab League and presented the plan, told reporters in Moscow that he'd 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 U.S. 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.

Anti-regime activist say more than 35,000 people have been killed since the anti-Assad uprising started in March 2011.

The holiday cease-fire was the first international effort in months to try to stop the violence, and it accomplished little.

The Britain-based Syrian Observatory for Human Rights, which compiles daily death tolls based on contacts inside Syria, said more than 500 people had been killed during the four-day holiday. It said more than 80 people were killed Monday and that the number was likely to rise further. Prior to the holiday, about 150 people were reported dead each day.