Turkish forces and allied Syrian rebels meanwhile expelled the Islamic State group from the last stretch of the Syrian-Turkish border under its control, a monitor said.
Syrian state media said the army and allied forces had taken an area south of Aleppo, severing the sole route left into the eastern neighbourhoods held by the opposition.
"The armed forces in cooperation with their allies took full control of the military academy zone south of Aleppo and are clearing the remaining terrorists from the area," state television said, citing a military source.
It said the advance "cut all the supply and movement routes for terrorist groups from southern Aleppo province to the eastern neighbourhoods and Ramussa."
The development leaves about 250,000 people living in rebel-controlled parts of the city cut off from the outside world once again, and will raise new fears about a humanitarian crisis in Aleppo.
Once Syria's economic powerhouse, the city has been ravaged by the war that began with anti-government protests in March 2011.
It has been roughly divided between government control in the west and rebel control in the east since mid-2012, but in recent months regime forces slowly began to encircle the east.
In July, they severed the only road into the rebel neighbourhoods, the key Castello Road running from the Turkish border in the north, creating food and fuel shortages in the east.
The siege prompted international concern, with aid agencies urging 48-hour ceasefires to ensure humanitarian access.
US-Russia talks stumble
In early August, rebel forces including Al-Qaeda's former Syrian affiliate battled regime forces south of the city to open a new route to the east, through Ramussa district.
But in recent days regime forces backed by Syrian and Russian war planes launched a counter-offensive.
A key regime ally, Moscow began an aerial campaign in support of President Bashar al-Assad's government last September, even as it continued to publicly support efforts for a negotiated solution to the five-year war.
Earlier Sunday, hopes were raised that Moscow and Washington might be on the verge of announcing a deal to halt the bloodshed.
US President Barack Obama said both nations were working "around the clock" on a ceasefire, and a State Department official said a deal was close.
But the hopes evaporated later in the day, with a State Department official saying Russia had "walked back on some of the areas we thought we were agreed on."
Instead, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov are set to meet again on Monday in Hangzhou, China, where G20 leaders are gathered.
"We're going to review some ideas tonight, a couple things on these couple of tough issues, and come back together and see where we are," said Kerry.
"We're not going to rush," he said, stressing the importance of reaching a deal that was able "to try to get the job done".
IS expelled from border
Washington is opposed to Assad's government, but has been accused by the opposition of failing to deliver concrete support.
And despite several rounds of international negotiations, a solution to the conflict that has killed more than 290,000 people and displaced millions remains elusive.
The conflict has become increasingly complex, involving not only regime and rebels, but international backers on both sides, Kurdish forces, jihadists and now Turkey.
Ankara began an operation inside Syria on August 24, dispatching troops to battle both the Islamic State group but also Syrian Kurdish forces it accuses of "terrorism".
On Sunday, the Syrian Observatory for Human Rights monitor said rebels backed by Turkish tanks and warplanes had seized the last parts of the border held by IS from the extremist group.
"IS has lost its contact with the outside world after losing the remaining border villages," the Britain-based monitor said.
Much of the earlier work to expel IS from elsewhere on the border was done by the Syrian Kurdish YPG, working with the US-led coalition against the jihadist force.
But Ankara considers the YPG a "terrorist" group and has been alarmed by its expansion along the border, fearing the creation of a contiguous, semi-autonomous Kurdish region in northern Syria.
The loss of the Turkish border will deprive IS of a key transit point for recruits and supplies, though the group continues to hold territory in both Syria and Iraq.
