Activists say a tunnel explosion in the northern Syrian city of Aleppo has lightly damaged the wall of the city's famous citadel.

The Britain-based Syrian Observatory for Human Rights said Sunday the tunnel was dug by rebels before it was discovered by troops who detonated it.

The Observatory said part of the sprawling citadel's wall collapsed.

Aleppo-based activist Nazeer al-Khatib said about 2 meters (yards) of the wall collapsed.

Syria's state-run news agency SANA said "terrorists" blew up a tunnel, causing a wall of the citadel to collapse.

Aleppo's medieval citadel is an imposing city landmark that once swarmed with tourists.

The rebels have detonated several tunnels since last year, including one that destroyed the Carlton Hotel, known for its elegant architecture and its proximity to the citadel.