The only way into rebel-held parts of Aleppo has been cut by a Syrian government advance that put the road within firing range, two rebel sources and a group monitoring the conflict say.

The move puts about 250,000 people living there under siege.

Rebel groups were waging a fierce battle to retake the areas captured overnight near the Castello Road, the rebels said on Thursday.

"Currently nobody can get in or out of Aleppo," Zakaria Malahifji of the Aleppo-based rebel group Fastaqim told Reuters.

The advance by pro-government forces in the al-Malah Farms area northwest of Aleppo brought them to within 1km of the Castello road, the Syrian Observatory for Human Rights said.

Heavy aerial and artillery bombardment had at times made the Castello Road impassable in recent weeks, but Thursday's advance makes it even easier for government forces to hit, effectively cutting off the opposition-held sector of the divided city.

"(The road) was not risk-free, but now there is not a matter of danger, now the road is cut," Malahifji said.

A second rebel official with fighters in the area said: "All the factions sent reinforcements and are trying to take back the positions taken by the regime, but the situation is very bad, there was heavy regime air cover in the night."

The Observatory estimates that between 250,000 and 300,000 people live in opposition-held parts of Aleppo.

The Syrian army had on Wednesday announced a countrywide, 72-hour truce.

The Observatory said the truce was meant for the entire war-torn country.

The ceasefire coincided with the start of the Eid al-Fitr holiday, which marks the end of Islam's holy month of Ramadan.

"This is a fake ceasefire," said Mahmoud al-Shami, an activist based in Aleppo.

"This is a new manoeuvre by the regime to score military advances," al-Shami told DPA via a message.

There was no official comment in Damascus.