Air strikes hit the only road into rebel-held areas of Aleppo city on Sunday in the heaviest bombardment since February, a rebel official and monitors said, jeopardizing access to an area where around 300,000 Syrians live.
They said Russian warplanes had carried out the attacks on the Castello road, which was still open but dangerous. Defense officials from Syria's government and its ally Russia could not immediately be reached for comment.
The Syrian Observatory for Human Rights group said the road had been hit in a week of escalating air strikes, with Sunday's attack the most intense yet.
The city of Aleppo, about 30 miles (50 km) south of the Turkish border, is divided between the government and rebels fighting to topple Syrian President Bashar al-Assad.
A truce was brokered by the United States and Russia in February. But the agreement has since unraveled, with fighting and bombardment in Aleppo playing a big part in its collapse.
"From 1 a.m. until 10 a.m., Russian jets were not quiet on the Handarat-Castello front," said Zakaria Malahefji, an official in the rebel group Fastaqim that operates in the area.
The nearby Owaija area was also heavily hit, he said.
"A group [of fighters] stationed there was killed," he added.
Observatory Director Rami Abdulrahman said the raids had been happening for a week. "This is more intense than the last days," he added.
A Russian Defense Ministry statement issued on Saturday accused insurgents of firing missiles at nearby areas, identifying them as members of the al Qaeda-linked Nusra Front, which was not included in the truce.
Rebels fighting under the banner of the Free Syrian Army say Nusra has little or no presence in Aleppo city.
Russia deployed its air force to Syria last year to bomb in support of the Syrian military and its allies.
Rebels say they can distinguish Russian from Syrian warplanes by the accuracy and intensity of their bombing, the way they fly in squadrons, and the fact they carry out raids at night.
Syrian government forces have mounted several offensives aimed at encircling rebel-held eastern Aleppo but these have all failed to date.