There are reports of intense fighting very close to the only road linking rebel-held areas of the Syrian city of Aleppo with the rest of the country.
The initial truce was announced by the government on Wednesday in respect of the Eid al-Fitr, which marks the end of the Islamic month of fasting.
Rebels attacked Syrian government positions in the historic center of Aleppo on Monday in response to an offensive that cut a road leading into the opposition-held sector of the city, monitors and insurgents said. Government advances were also reported in Daraya, a besieged opposition-held town overlooking Damascus airport, the opposition-operated Facebook account of the Local Council of Daraya reported.
However, the rebels had so far not managed to make any gains because they had come under heavy bombardment by government aircraft.
Also on Thursday, government forces cut the only way into militant-held parts of Aleppo, severing the terrorists' only lifeline from the city to the outside world.
"Currently, nobody can get in or out of Aleppo", Zakaria Malahifji, of the Aleppo-based rebel group Fastaqim, said.
The Syrian military has extended a nationwide ceasefire that expired on Friday for another 72 hours, state media reported on Saturday. Groups fighting under the banner of the Free Syrian Army (FSA) say however that they control the rebel-held part of the city.
Rebel forces responded by firing barrages of rockets into the government-held west of the city on Friday, killing at least 41 people, majority civilians, according to the Syrian Observatory for Human Rights.
"We declare we will abide by it so long as the other side does the same", said the statement, posted on the Twitter account of a prominent Syrian rebel figure.
It was not clear whether cease-fire included areas under Al-Nusra control.
Meanwhile in the al Qaeda-held town of Darkush, near the Turkish border, at least 22 civilians were killed and dozens wounded by air strikes, the Observatory said.
More than 280,000 people have been killed and millions displaced since Syria's civil war erupted with the brutal repression of anti-government protests in 2011.