At least 28 civilians were killed on Friday in a series of airstrikes targeting rebel-held areas in Syria's northern city of Aleppo, a monitoring group reported.
The dead included eight people killed in a strike on a bus that was travelling on the opposition-controlled Castello road in Aleppo, the Syrian Observatory for Human Rights said.
The road is strategically important because it is the only access point to rebel-held areas in Aleppo.
The bombardment was believed to have been carried out by warplanes belonging to the Syrian regime or its ally, Russia, added the Britain-based watchdog, which relies on a network of activists inside Syria.
The Castello road is now "effectively cut" due to the repeated airstrikes, the Observatory’s head, Rami Abdel-Rahman, said.
Videos posted by activists online showed both destroyed buildings and victims being pulled from rubble, though it was not clear if the images of the people were from today.
Aleppo’s opposition-held areas have been the target of at least 18 airstrikes since early Friday, according to the Observatory.
There was no official comment in Damascus.
In recent months, government forces and rebels have intensified retaliatory attacks in Aleppo.
At least 436 civilians, including 97 children, have been killed in these assaults since April 22, the Observatory said.
Once Syria's commercial hub, Aleppo is now divided between the government-held west and the rebel-controlled east.