Iraq's military said ISIS destroyed the 12th century al-Nuri mosque in Mosul's Old City, where ISIS fighters remain.The medieval mosque, with its tall leaning al-Hadba minaret, was the site where ISIS leader Abu Bakr al-Baghdadi made a rare public appearance and in July 2014 declared the group's "caliphate" in Iraq and Syria."Daesh's bombing of the al-Hadba minaret and the al-Nuri Mosque is a formal declaration of their defeat," Iraqi Prime Minister Haider al-Abadi tweeted, according to an Associated Press translation. Daesh is an Arabic acronym for the Islamic State group.Iraq's military said ISIS fighters used explosives to blow up the more than 800-year-old mosque, the AP reports."This is a crime against the people of Mosul and all of Iraq, and is an example of why this brutal organization must be annihilated," Maj. Gen. Joseph Martin, who commands coalition ground forces in Iraq, said in a statement.ISIS media arm Amaq claimed a U.S. airstrike was responsible for the destruction. U.S. Army Col. Ryan Dillon told the AP that the U.S. "did not conduct strikes in that area at that time."The Iraqi military said its counterterrorism forces were within 50 meters of the mosque when it was destroyed. A video on social media shows what it says is the explosion.Reuters provided some historical background for the al-Nuri mosque: