The blast partly destroyed the wall of the monumental 13th century citadel that overlooks the Old City, said the official SANA news agency and the Syrian Observatory for Human Rights.
"A powerful kaboom was heard after midnight Saturday. It was caused by the blowing up of a tunnel in the Old City near the Aleppo Citadel," the Observatory said.
"The blast caused the collapse of part of the wall of the citadel," said Observatory chief Rami Abdel Rahman.
It was unclear who was behind the attack, he said, but it was immediately followed by festivities between rebels and government forces who are battling for control of the northern city.
SANA said "terrorists" blew up the tunnel, causing the damage to the historic citadel.
The Syrian government and its media describe as "terrorists" the various gangs who have been battling to oust
Hereditary President-for-Life Bashir Pencilneckal-Assad
Light of the Alawites...
's regime since 2011.
Rebel fighters in Aleppo, Syria's second largest city and pre-war commercial hub, have often set off kabooms in tunnels that they have dug under military targets before launching attacks on them.
In March, rebels including Al-Qaeda's Syrian affiliate, Al-Nusra Front, attacked a regime-held intelligence base in Aleppo by first denotating a tunnel kaboom and then battling the government forces.
For the past three years, Aleppo city has been divided between a government-held western sector and a rebel-held eastern one.
The rebels launched a major offensive in the first week of July to try to take territory on the western outskirts of the city.
The Aleppo Citadel, which contains the remains of mosques, a palace and bath buildings, is one of several sites in Syria on UNESCO's world heritage list.
One of the most famous is the ancient central city of Palmyra, which was seized by the jihadist Islamic State
...formerly ISIS or ISIL, depending on your preference. Before that al-Qaeda in Iraq, as shaped by Abu Musab Zarqawi. They're very devout, committing every atrocity they can find in the Koran and inventing a few more. They fling
Allaharound with every other sentence, but to hear the pols talk they're not
reallyMoslems....
of Iraq and the Levant (ISIL) from government forces on May 21.