Russian warplanes are reported to have carried out heavy air strikes on a key road leading to rebel-held areas of Aleppo, Syria’s biggest city.
The Syrian Observatory for Human Rights said the air strikes on May 22 targeted 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.
Observatory Director Rami Abdulrahman said the raids had been happening for a week. "This is more intense than the last days," he added.
The city of Aleppo, about 50 kilometers 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.
A Russian defense Ministry statement issued on May 21 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.
Mainstream Syrian rebel groups said on May 22 they would no longer abide by the UN truce deal unless the Syrian army ended a major assault on their positions in the suburbs of Damascus within 48 hours.
A statement by the Free Syrian Armysigned by nearly 40 rebel groups said they would deem the cessation of hostilities deal as having "totally collapsed" if the assault by Syrian government and allied Lebanese Hizballah forces fighters did not cease within two days.
In their joint statement, the rebels said the continued attacks by the army on the besieged rebel-held areas around Damascus and their strongholds in Aleppo city and Idlib Province were putting peace-making efforts at risk.