Syrian government forces have effectively cut the only road into rebel-held areas of the city of Aleppo, military sources and rebels say.
After heavy fighting and air strikes overnight, troops advanced to within 1km (0.6 miles) of the Castello Road, within range of small arms fire.
Rebels said that meant no-one could now get into or out of the east of the city, home to up to 300,000 people.
The assault came hours after the government declared a 72-hour truce.
Aleppo, once Syria's commercial and industrial hub, has been divided in roughly two since 2012, with the government controlling the western half and rebel factions holding the east.
The Syrian Observatory for Human Rights, a UK-based group that monitors the five-year conflict through a network of sources, said clashes between government forces and Islamist rebels had erupted around north-western Aleppo after midnight.
With extensive air support, government troops and allied militiamen subsequently moved into the southern al-Mallah Farms area and took control of a mosque complex that overlooks the Castello Road, it added.
The advance meant the road was within range of their medium and light weapons, according to the Syrian Observatory.
The official Sana news agency cited a military source as saying that army fire had "cut" the Castello Road and that units were now "hunting down the fleeing terrorist groups in the area".
"Currently nobody can get in or out of Aleppo," Zakaria Malahifji of the Fastaqim rebel group told the Reuters news agency.
Heavy air strikes and artillery bombardment had made the Castello Road very difficult to travel in recent weeks, but now it was easier for government forces to target vehicles using it, he said.
"[The road] was not risk-free, but now there is not a matter of danger, now the road is cut," he added.
Another rebel fighter told Reuters: "All the factions sent reinforcements and are trying to take back the positions taken by the regime, but the situation is very bad."
Jaysh al-Islam said four of its fighters had been killed in the fighting, while the Syrian Observatory said rebel fire on the government-held Saif al-Dawla district had left three people dead, the AFP news agency reported.
The Syrian war and Aleppo
March 2011: Anti-government protests erupt across Syria, but Aleppo is initially untouched as a result of a state crackdown
February 2012: As the rebellion turns into a conflict, armed clashes are reported with increasing frequency in Aleppo province
July 2012: The battle for Aleppo begins. Rebels make swift advances, but are unable to consolidate their gains and the city becomes divided
2013: The government begins bombarding rebel districts with barrel bombs, causing thousands of casualties
September 2015: Government launches fresh offensive on rebel-held east in the wake of Russia's intervention in the conflict
February 2016: The government captures towns north-west of Aleppo, threatening to encircle the rebel-held area of the city