Nearly 50 people were killed on Thursday when Syrian rebels attacked one of the largest military barracks in the country, in northern Aleppo, the Syrian Observatory for Human Rights said.
It said at least 27 soldiers and pro-regime militiamen were killed and that the rebel losses of 20 dead included a commander.
"Rebels, including fighters from Al-Nusra Front and the Islamic Front, launched an assault today on the barracks in Hanano in Aleppo," Observatory director Rami Abdel Rahman said.
State media, meanwhile, reported that the army had "foiled an attempt by terrorist groups to infiltrate the barracks" and killed a number of them.
President Bashar al-Assad's regime refers to rebels battling to topple it as "terrorist" groups.
Abdel Rahman said the barracks is one of the largest in Syria.
"It's strategically important because it's on a hill that overlooks parts of northern Aleppo," he added.
Once Syria's economic hub, Aleppo has been divided between regime control in the west and rebel control in the east since shortly after combat began there in mid-2012.
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Abdel Rahman said the attack began when "rebels detonated explosives in tunnels they had dug beneath army positions around the barracks".
State television also reported that the rebels had "detonated explosives in three tunnels around the barracks".
Abdel Rahman said the barracks overlooks a key supply route for rebels going north into the rest of Aleppo province.
Regime forces have advanced around some of the eastern outskirts of Aleppo city, reopening its international airport to the east.
The air force has also waged a relentless campaign against eastern parts of the city, including dropping explosives-packed "barrel bombs".
Rebels respond by firing rockets into pro-regime areas.
The Observatory said that such rocket attacks killed at least 11 civilians, including two women and a child, on Thursday and wounded 40.
In the Zahra district in the west of the city, rebels have seized buildings near the air force intelligence headquarters, it said.
Fighting has raged in the area since April 12, with rebels led by Al-Nusra battling to take the key target.