Syrian jets bombed suburbs of Damascus and a car bomb killed 10 people in the capital on Monday, on the last day of a four-day truce which United Nations chief Ban Ki-moon acknowledged had failed.
Each side blamed the other for breaching the Eid al-Adha truce arranged by international envoy Lakhdar Brahimi, who nevertheless promised to pursue his peace efforts.
"I am deeply disappointed that the parties failed to respect the call to suspend fighting," Ban said in Seoul, where he was visiting to receive the Seoul Peace Prize.
"This crisis cannot be solved with more weapons and bloodshed... the guns must fall silent," he said.
Brahimi, after meeting Russian Foreign Minister Sergei Lavrov in Moscow, voiced regret that the cease-fire had not worked better. Asked whether UN peacekeepers might be sent to Syria, he said there was no immediate plan for that.
Although President Bashar Assad's government and several rebel groups accepted the plan to stop shooting over the Muslim religious holiday, it failed to stem the bloodshed in a 19-month-old conflict that has already cost at least 32,000 lives.
According to the Syrian Observatory for Human Rights, an opposition watchdog, 420 people have been killed since Friday.
Damascus residents reported heavy air raids on the suburbs of Qaboun, Zamalka and Irbin overnight and on Monday which they said were the fiercest since jets and helicopters first bombarded pro-opposition parts of the Syrian capital in August.
Syrian state television said women and children were among those killed by a "terrorist car bomb" near a bakery in Jaramana, in the southeast of Damascus. Damascus residents say the district is controlled by Assad loyalists.
A Syrian government official said the blast on Monday in Jaramana also wounded 41 people and caused heavy damage. He spoke on condition of anonymity because he was not authorized to speak to the media.
State media said Assad's armed opponents had broken the truce throughout the Eid.
"For the fourth consecutive day, the armed terrorist groups in Deir al-Zor continued violating the declaration on suspending military operations which the armed forces have committed to," state news said, later adding that rebels had attacked government forces in Aleppo and the central city of Homs.
The Damascus air raids followed what residents said were failed attempts by troops to storm eastern parts of the city.
"Tanks are deployed around Harat al-Shwam [district] but they haven't been able to go in. They tried a week ago," said an activist who lives near the area and who asked not to be named.
Brahimi, who will visit Beijing after Moscow, said the renewed violence in Syria would not discourage him.
"We think this civil war must end... and the new Syria has to be built by all its sons," he said. "The support of Russia and other members of the Security Council is indispensable."
Russia and China have vetoed three Western-backed UN draft resolutions condemning Assad's government for the violence.
Beijing has been keen to show it does not take sides in Syria and has urged the government there to talk to the opposition and take steps to meet demands for political change. It has said a transitional government should be formed.
Big-power rifts have paralyzed United Nations action over Syria, but Assad's political and armed opponents are also deeply divided, a problem which their Western allies say has complicated efforts to provide greater support.
Syrian opposition figures, including Free Syrian Army commanders, started three days of talks in Istanbul on Monday in the latest attempt to unite the disparate groups.
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