Syrian jets bombed parts of Damascus on Monday in what residents said were the capital's fiercest air raids yet, at the end of what was supposed to be a four-day truce.
"More than 100 buildings have been destroyed, some leveled to the ground," said opposition activist Moaz al-Shami. "Whole neighbourhoods are deserted."
Each side in the 19-month-old conflict between President Bashar al-Assad and rebels blamed the other for breaking the truce proposed by peace envoy Lakhdar Brahimi to mark a Muslim holiday. Two car bombs rocked the capital on Monday, state media reported.
"I am deeply disappointed that the parties failed to respect the call to suspend fighting," U.N. chief Ban Ki-moon said.
"This crisis cannot be solved with more weapons and bloodshed ... the guns must fall silent."
Although the military and several rebel groups accepted the plan to stop shooting over Eid al-Adha, which ends on Monday, 500 people have been killed since Friday, according to the Syrian Observatory for Human Rights, an opposition organization.
Damascus residents said Monday's air raids were the heaviest since jets and helicopters first bombarded pro-opposition parts of the capital in August.
"Even electricity poles have been hit and they are lying among pools of water from burst pipes. There is no food, water, electricity or telephones," said Shami, who said he witnessed three air raids in the northeastern suburb of Harasta alone.
State media said "armed terrorist groups" had broken the truce over the four days in the cities of Aleppo, Homs and Deir al-Zor and had detonated two car bombs in the capital on Monday.
One killed 10 people, including women and children, near a bakery in Jaramana, a district controlled by forces loyal to Assad. The other was in Hajar al-Aswad, a neighborhood where rebels are based.
The conflict - which pits majority Sunni Muslims against a leadership dominated by Alawites - a branch of Shi'ite Islam - has grown increasingly sectarian.
The Observatory said that more than 200 Kurdish civilians were detained over the weekend by "militants" and a Kurdish man died from wounds he sustained during torture.
Rebels in Aleppo have fought with Kurdish militants in recent days, accusing Syria's Kurds of siding with Assad. Many Kurds say they want to stay out of the violence by distancing themselves from either side.
Brahimi, who met Russian Foreign Minister Sergei Lavrov in Moscow on Monday before flying to Beijing, said the renewed violence 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 (U.N.) Security Council is indispensable."
Russia and China have vetoed three Western-backed U.N. draft resolutions condemning Assad's government for the violence.
Beijing, keen to show it does not take sides in Syria, has urged Damascus to talk to the opposition and meet demands for political change and has advocated a transitional government.
Big-power rifts have paralyzed U.N. 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.
The Syrian Foreign Ministry released a statement after Monday's car bombs, lambasting the Security Council for not condemning actions it said "encouraged terrorists to continue their crimes against the Syrian people."
The civil war continued to spill over Syria's borders on Monday, as mortar bombs landed in southern Turkey. A judicial source in Lebanon said eight Syrians were arrested near the border in possession of arms and one was charged with firing at the Lebanese army.
Additional reporting by Thomas Grove in Moscow and Michael Martina in Beijing; Writing by Oliver Holmes; Editing by Robin Pomeroy