The UN Security Council has unanimously approved a resolution demanding a 30-day ceasefire in Syria to allow aid deliveries and medical evacuations.
However, some of the biggest jihadist rebel groups, and their associates, are not covered by the truce, raising questions about its real impact.
The Eastern Ghouta rebel enclave near Damascus has been bombarded by government forces for the past week.
After the vote in New York, activists said air strikes were continuing.
The vote had been delayed several times since Thursday as members struggled to come to an agreement.
Russia, an ally of Syria's government, wanted changes, while Western diplomats accused Moscow of stalling for time.
Some 500 people are said to have been killed by government forces in the enclave since last Sunday while rebels fire on Damascus has reportedly killed at least 16 civilians.
The draft had said the ceasefire would not apply to operations against the Islamic State (IS) group, al-Qaeda and the Nusra Front.
The Nusra Front is a former al-Qaeda affiliate in Syria which changed its name to Hayat Tahrir al-Sham (HTS) when it formed an alliance with other militants last year.
The US ambassador to the UN, Nikki Haley, called for the ceasefire to be implemented immediately but said she was sceptical that Syria would comply.
On Saturday, the UK-based Syrian Observatory for Human Rights said at least 29 civilians had been killed, including 17 in the main town, Douma.
The group said the strikes were being carried out by both Syrian and Russian planes although Russia denies direct involvement.
Barrel bombs and shells have been dropped on the area, where some 393,000 people remain trapped.
The Syrian government says its attempts to recapture Eastern Ghouta are directly due to the HTS presence there.