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.
But in a change, Rami Abdul-Rahman, head of the Observatory, said the number of airstrikes spiked on Monday.
"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.