Syria government strikes kill 31 in and near Aleppo: White Helmets

Government bombing raids in and around the northern Syrian city of Aleppo killed 31 civilians on Friday, including 10 when their bus was hit, the civil defence said in a new toll.

"As well as the 10 civilians killed in strikes on the bus... 21 others died in intense strikes on several neighbourhoods in the east of the city since dawn," the organisation known as the White Helmets said.

An AFP correspondent in Aleppo said Friday's bombing raids were the most intense in more than a week, with dozens of barrel bombs - crude, unguided explosive devices - hitting several eastern quarters of the city.

The Britain-based Syrian Observatory for Human Rights reported intense strikes on the Castello road - a key rebel supply route out of divided Aleppo - giving a toll of eight dead civilians.

A bus on the road was also hit on Wednesday, resulting in seven civilian deaths.

Observatory director Rami Abdel Rahman said the Castello road, which for civilians in rebel-held areas is the route out of Aleppo, was now "effectively cut".

"All movement is targeted, be that buses or bystanders," he said.

Abdel Rahman said rocket fire on government-held areas of the city overnight caused several casualties, but he was unable to provide an exact toll.

A truce agreed by Russia and the US in February has been violated nearly continuously around Aleppo, where the government and rebel groups have fought for control since 2012.

More than 300 civilians have been killed in Aleppo since April as rebels have pounded government-controlled neighbourhoods with rocket and artillery fire and the government has hit rebel areas with air raids.

The UN no longer keeps track of the war's death toll, citing the inaccessibility of many locations and inability to verify information. 

However, in April, UN envoy to Syria Staffan de Mistura estimated that 400,000 people had been killed since the fighting started in 2011.