At least 20 Syrian regime forces were killed Saturday when Islamist rebels planted explosives in a tunnel under an army position in the northern city of Aleppo, a monitor said.
Aleppo's historic Old City has seen horrific violence ever since the rebels launched a major offensive against Syria's former economic hub in July 2012.
The area is a flashpoint area that sees daily fighting, and the army has set up multiple positions there.
"Islamist rebels detonated a tunnel near the Zahrawi market in the Old City of Aleppo, killing at least 20 army soldiers and pro-regime militiamen," said the Syrian Observatory for Human Rights.
Fighting broke out after the explosion, and at least one rebel was killed, said the Britain-based Observatory.
The attack was claimed by the Islamic Front, Syria's largest rebel alliance which groups thousands of fighters across the strife-torn country.
It posted a link to a video on its Twitter account, showing a huge blast throwing a massive cloud of debris up into the air.
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In recent weeks, the Islamic Front has frequently used tunnels to plant massive amounts of explosives beneath army positions.
The tactic has been used mainly in Aleppo and neighbouring Idlib provinces.
The latest blast came a day after the Observatory said some 2,000 people have been killed since January in regime bombing of rebel-held areas of Aleppo city and nearby towns and villages.
The Syrian Revolution General Commission, a network of activists on the ground, said the air force launched fresh barrel bomb attacks on the Beni Zeid area of Aleppo on Saturday.
Rebels, meanwhile, fired mortar rounds on a neighbourhood controlled by the regime in Aleppo, killing four people and wounding 17 others, said the Observatory.
An activist also reported that the Red Crescent and the International Committee of the Red Cross delivered 14 trucks of food aid Saturday to Raqa province, much of which held by radical Islamic State of Iraq and the Levant.
"The aid that arrived is by no means sufficient to feed the families here, who suffer from terrible poverty," said activist Abu Ibrahim from Raqa.
Syria's war has killed more than 162,000 people, and forced nearly half the population to flee their homes.