A section of the wall of the ancient citadel in Aleppo was destroyed by an explosion in a tunnel under the city, state media and activists reported on Sunday.
The Syrian Observatory for Human Rights said the damage occurred when government forces blew up a tunnel dug by insurgents under the city, while the state news agency said the tunnel was blown up by rebels.
A section of the wall was shown turned to rubble in footage posted online by Halab News Network, an activist outlet.
The Ancient City of Aleppo was added to the UNESCO list of World Heritage in danger in 2013.
Aleppo was Syria's most populous city before the eruption of the civil war. It is divided into areas controlled separately by the government and insurgents who are battling to topple President Bashar al-Assad.