IRBIL, Iraq (AP) — The Islamic State group blew up a historic landmark in Mosul — the city’s famed 12th century al-Nuri mosque with its iconic leaning minaret known as al-Hadba, from where the IS leader proclaimed the militant group’s self-styled caliphate nearly three years ago.

The explosion destroyed another piece of priceless Iraqi cultural heritage but also sent a strong message to U.S.-led coalition forces and Iraqi troops closing in on the last stronghold of IS, in Mosul’s Old City neighborhood.

Iraq’s Ministry of Defense said the militants detonated explosives planted inside the structures on Wednesday night. Prime Minister Haider al-Abadi tweeted early on Thursday that the destruction was an admission by the militants that they are losing the fight for Iraq’s second-largest city.

“Daesh’s bombing of the al-Hadba minaret and the al-Nuri Mosque is a formal declaration of their defeat,” al-Abadi said, using the Arabic acronym for the Islamic State group.

The al-Nuri mosque, which is also known as Mosul’s Great Mosque, is where IS leader Abu Bakr al-Baghdadi made a rare public appearance, declaring a so-called Islamic caliphate in the summer of 2014, shortly after Mosul was overrun by the militants. The minaret that leaned like Italy’s Tower of Pisa had stood for more than 840 years.