Airstrikes targeted one of the last roads into opposition-held areas of Aleppo on Tuesday, the Syrian Observatory for Human Rights said, a day after the United States and Russia presented a cease-fire plan to begin Saturday.
Observatory Director Rami Abdulrahman described the airstrikes as heavy and said they were believed to be carried out by Russian jets.
The targeted road links rebel-held parts of Aleppo with rural areas to the west of the city that provide access to other opposition-held areas, including Idlib province.
A Syrian Army offensive backed by Russian airstrikes and allied fighters from Lebanon and Iran earlier this month cut the most direct rebel supply route from opposition-held parts of Aleppo to the northern border with Turkey.
Government offensives are also targeting areas held by insurgents to the south of the city, and areas held by the Islamic State group to the east of the city.