Islamic State extremists on June 21 blew up Mosul’s Grand Al-Nuri Mosque and its iconic leaning minaret in what the leader of Iraq called “an official declaration of defeat.”

The mosque, built in 1172, and its minaret known as “the hunchback” were chosen as the venue where IS leader Abu Bakr al-Baghdadi in July 2014 declared himself “caliph” of what he hoped would be a militant empire growing out of Iraq and Syria.

His black flag had flown over the mosque’s famous minaret for three years, and its destruction now marks the group’s concession that it is near defeat, Iraqi Prime Minister Haidar al-Abadi said afterwards.

Before the blast, Iraqi forces had encircled IS’s stronghold in the Old City, the last district under the militants’ control in Mosul,and had been making slow but steady progress toward retaking control of the heavily populated area.

U.S. Army Major General Joseph Martin, a senior U.S. commander in the battle against IS, called the mosque’s devastation “a crime against the people of Mosul and all of Iraq” and “an example of why this brutal organization must be annihilated.”

Iraq’s military said the militants blew up the mosque as Iraqi forces were advancing toward targets deep in the Old City and got to within 50 meters of the mosque. It called the move a “historical crime.”

The destruction of two of Mosul’s best-known landmarks adds to a long list of world heritage sites the militants have destroyed in Iraq and Syria, including dozens of sites in Mosul and many of Palmyra’s famed monuments.

IS militants who seized control of Mosul in June 2014 had previously targeted the minaret, which Iraqis lovingly called Al-Hadba, or “the hunchback,” because it listed like the leaning Tower of Pisa in Italy.

But residents of Mosul protected the mosque at that time by creating a human chain around it.

Iraqi officials had privately expressed the hope that the mosque could be captured in time for Eid al-Fitr, the festival marking the end of Ramadan, the Muslim fasting month. The first day of the Eid falls this year on June 25 or 26 in Iraq.

But because of the mosque’s close association with Baghdadi’s attempt at establishing a caliphate, Mosul residents told reporters they feared it would be targeted if the group ever were on the verge of losing control over the city.

IS’s Amaq news site claimed after the blast that the mosque was destroyed by a U.S. air strike, but that claim was quickly denied by the anti-IS coalition, which said no strikes were carried out in the area on June 21.

The mosque was named after Nuruddin al-Zanki, a nobleman who fought the early crusaders from a fiefdom that covered territory in modern-day Turkey, Syria, and Iraq.

It was built with seven bands of decorative brickwork in complex geometric patterns ascending in levels towards the top, similar to designs found in Persia and Central Asia.

The minaret started listing centuries ago and was long considered an endangered monument.

With reporting by Reuters, AP, AFP, and dpa