Syrian government forces are reported to have broken a siege of two villages north-west of Aleppo, severing a key rebel supply route into the city.
State TV said troops and allied militiamen, backed by Russian air strikes, had reached Nubul and Zahraa.
A military source told the AFP news agency that opposition-controlled areas of Aleppo were now cut off from Turkey, to the north, which backs the rebels.
The offensive has threatened to derail UN efforts to convene peace talks.
The umbrella group representing opposition factions in Geneva pulled out of a meeting with UN special envoy Staffan de Mistura on Tuesday afternoon, after what they described as "unprecedented" Russian bombardment of rebel-held areas around Aleppo.
"The level of trust is close to zero between the two sides," Mr de Mistura warned.
More than 250,000 people have died in almost five years of war in Syria.
Eleven million others have fled their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other, as well as jihadist militants from so-called Islamic State (IS).

BBC