With Russia's backing, the UN Security Council unanimously backed a 30-day ceasefire in Syria to allow for humanitarian aid deliveries and medical evacuations.
The resolution demanding the ceasefire "without delay" was adopted this evening as Syrian government forces pounded the rebel-held enclave of Eastern Ghouta.
The two major rebel factions in Eastern Ghouta welcomed the UN resolution.
In separate statements, Jaish al-Islam and Failaq al-Rahman pledged to protect aid convoys that come into the besieged rebel enclave near Damascus.
The insurgents said they would commit to a truce, but would respond to any violation by the Syrian government and its allies.
More than 460 civilians, including more than 100 children, have been killed in the seven-day offensive on the rebel-held enclave, where UN Secretary-General Antonio Guterres said 400,000 Syrians are living in "hell on Earth".
Negotiations ran into problems over a key provision of the text that specified when the ceasefire would begin.
Negotiations also stumbled over Russian demands that rebel groups fighting President Bashar al-Assad's forces comply with the truce.
Rescuers in Eastern Ghouta said the bombing would not let up long enough for them to count the bodies, in one of the bloodiest air assaults of the seven-year war.
Warplanes pounded the rebel enclave today, in a fierce escalation by Damascus and its allies, an emergency service, a witness and a monitoring group said.
Residents holed up in basements and medical charities decried attacks on a dozen hospitals, the only big rebel bastion near the capital.
The Damascus government and Russia, its key ally, say they only target militants. They have said they seek to stop mortar attacks injuring dozens in the capital, and have accused insurgents in Ghouta of holding people as human shields.
There was no immediate comment from the Syrian military.
A surge of rocket fire, shelling and air strikes has killed over 500 people since last Sunday night, the Syrian Observatory for Human Rights said. The dead included more than 120 children.
The Britain-based monitor said raids hit Douma, Zamalka and other towns there today, killing 31 people.
First responders searched for survivors after strikes on Kafr Batna, Douma and Harasta, the Civil Defence in Eastern Ghouta said. The rescue service, which operates in rebel territory, said it had documented at least 350 deaths in four days earlier this week.
"Maybe there are many more," said Siraj Mahmoud, a civil defence spokesman in the suburbs. "We weren't able to count the martyrs yesterday because the warplanes are touring the skies."
As the bombs rain down, some hitting emergency centers and vehicles, workers have struggled to pull people from the rubble, Mr Mahmoud said.
"But if we have to go out running on our legs and dig with our hands to rescue the people, we will still be here," he said.
The United Nations says nearly 400,000 people live in Eastern Ghouta, a pocket of satellite towns and farms under government siege since 2013, without enough food or medicine.