Syrian government air strikes have killed dozens of civilians in and around the northern city of Aleppo, volunteer rescuers have said.

At least 28 people were killed in raids on several neighbourhoods in the rebel-held east of Aleppo, the civil defence, also known as the White Helmets, said on Friday.

Dozens of barrel bombs - crude, unguided explosive devices – were dropped in the strikes, described by an AFP news agency correspondent as the most intense in more than a week.

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Ten more people were killed when a bus they were travelling in was hit on Castello road, a key rebel supply route out of Aleppo, the White Helmets added.

The Syrian Observatory for Human Rights said Castello road was now "effectively cut".

"All movement is targeted, be that buses or bystanders," Rami Abdel Rahman, the head of the UK-based monitoring group, said.

'Nowhere to go'

Separately, the Observatory said on Friday that the US had air dropped weaponsto rebel fighters in Aleppo province, who have been battling the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

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In recent days, fighting has intensified near Marea, with the United Nations sounding a warning over the fate of an estimated 8,000 Syrians trapped by the violence.

Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep, on the Turkish side of the Turkey-Syria border, said thousands of people stranded in Aleppo and elsewhere have nowhere to go as fighting rages on.

"They can't go back to urban areas because they are afraid of the intensified air strikes by the Russians and the Syrian government, and they can't go to the border with Turkey because Turkey has been sealing its borders," he said.

"It's a very delicate situation."

Later on Friday, the UN said it would ask permission from the Syrian government on Sunday to airdrop or airlift humanitarian aid to besieged areas.

During a closed-door meeting of the Security Council, diplomats described airdrops as a "last resort" to reach thousands of civilians in need of aid.

Source: Al Jazeera and agencies