At least 40 Syrian soldiers were killed on Saturday when rebels detonated explosives packed beneath an army base in Aleppo, activists and rebels said.
The Islamic Front, an umbrella rebel organisation, claimed responsibility for the bombing, though the claim could not be immediately verified.
A video posted on the internet showed a massive blast sending clouds of dust and debris into the air as gunshots rang out in the old Zahrawi area of Aleppo.
The front said 40 government soldiers were killed. The UK-based Syrian Observatory for Human Rights said more than 20 died in the blast and added that fierce clashes had erupted along the divided city's frontlines, where fighting between rebels and troops has escalated in recent days.
Rebels fighting to overthrow President Bashar al-Assad regularly carry out guerrilla attacks but have only recently begun using large tunnel bombs on military targets. Last week they claimed responsibility for blowing up a hillside army base and a hotel used by soldiers in Aleppo.
The government has vastly superior firepower and has intensified air strikes using crude barrel bombs on residential areas in rebel-held Aleppo, killing at least 132 civilians in the last three days, a local medical group said.
Gunbattles, air strikes, car bombs, shelling and executions regularly kill more than 200 people a day in Syria, where a conflict that started as a peaceful protest movement has killed over 150,000 people and forced millions from their homes.
Despite the carnage and loss of swathes of territory in the north and east to insurgents, Syria plans to hold a presidential election next Tuesday that is all but certain to give Assad a third term. Opponents have dismissed the vote as a farce.