A new wave of airstrikes has hit Syria's eastern Ghouta ahead of a United Nations Security Council vote to demand a 30-day ceasefire across the country.

For a sixth consequent day, warplanes raided the populated agricultural pocket east of the capital, the last rebel bastion near Damascus. 

According the Syrian Observatory for Human Rights monitoring group, the increase in airstrikes has killed at least 417 people and injured hundreds more. 

Medical employees said jets have hit more than a dozen hospitals, making it near impossible to treat the wounded.

The Britain-based monitoring group said Syrian government warplanes and artillery hit Douma, Zamalka, and other towns across the enclave in the early hours on Friday. 

A witness in Douma, who asked not to be identified, said the early morning bombing was the most intense so far. 

The bombing of eastern Ghouta in Syria since Sunday night has been among the fiercest of the war.

The UN envoy for Syria has pleaded for a truce to halt one of the worst air assaults of the seven-year war and prevent a "massacre". The UN Security Council was considering a resolution, which Kuwait and Sweden drafted, demanding "a cessation of hostilities throughout Syria for all

military operations" for 30 days to allow aid deliveries and medical evacuations.

The resolution does not cover Daesh, al-Qaeda and the Nusra Front, which Moscow and Damascus say they have targeted in eastern Ghouta.