Syrian government forces are reported to have broken a siege of two towns 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 threatens to derail UN efforts to start peace talks in Geneva.
Basma Kodmani, a member of the umbrella group representing opposition factions, the High Negotiations Committee, said the government's encirclement of Aleppo was a "horrible development" that sent the message that "there is nothing to negotiate. Just go home."
However she added, in an interview with the Associated Press news agency: "We're not going home."
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 so-called Islamic State.
Syria's state news agency, Sana, reported that the siege of Nubil and Zahra was broken on Wednesday by the army and the pro-government Popular Committees.
The two mainly Shia towns, which have a population of about 40,000, have been besieged by hardline Sunni Islamist and jihadist rebels since early 2013.
The television station of Lebanon's Hezbollah movement, which has sent thousands of fighters to Syria to bolster Mr Assad, also reported on the advance and broadcast footage of the fighting.
In the course of breaking the siege, other army units seized territory between the nearby villages of Mayer and Muarrasat al-Khan, cutting off the rebel supply route between Turkey and Aleppo, Sana said.
The Syrian Observatory for Human Rights, a UK-based monitoring group, reported that "heavy" air strikes by Russian warplanes had assisted the advance.
Earlier, it said there had been more than 320 Russian air strikes in the Aleppo area since Monday morning, and that at least 18 civilians were killed on Tuesday alone.
A UN spokeswoman told Reuters news agency that the offensive had uprooted hundreds of families in the towns of Bayanoun, Hariyatan, Anadan, Hayan and Rityan, and left three humanitarian aid workers dead.
The BBC's Rami Ruhayem in Damascus says that if their gains are confirmed, government forces will be in the position to expand their reach throughout Aleppo, one of the last remaining strongholds for the rebels.
It is one of the most significant advances for the regime since the Russian military intervention in September, which has had a decisive impact on the ground in several areas throughout Syria, our correspondent adds.
The opposition HNC delegation pulled out of a meeting with UN special envoy Staffan de Mistura in Geneva on Tuesday afternoon after what it described as the "unprecedented" Russian bombardment around Aleppo.
But Russian Foreign Minister Sergei Lavrov stressed that it would not stop the air strikes "until we really defeat terrorist organisations like al-Nusra Front". The Syrian affiliate of al-Qaeda is part of the rebel alliance that besieged Nubl and Zahraa.