Great Mosque of al-Nuri in western Mosul Destroyed

This aerial view taken on June 21, 2017 and
provided by Iraq's Joint Operation Command
 shows destruction inside Mosul's Nuri mosque
 compound CREDIT: AFP

The same area in August 2008 (Google  Earth)

The Islamic State has blown up The Great Mosque of al-Nuri with its distinctive leaning minaret in western Mosul, according to U.S. and Iraqi forces (Alex Lubben, 'ISIS blows up 845-year-old mosque, tries to blame U.S.' vice.com,  Jun 21, 2017). The twelfth-century mosque, along with its minaret, was one of Iraq's most famous buildings. Haider al-Abadi, Iraq's prime minister, said the destruction of the sites was "an official declaration of defeat" by Isil in the eight-month-old battle for Mosul. Probably the aim was to deny the government the possibility of declaring victory there. The destruction of the mosque (apparently by explosives previously placed inside it) has also beebn taken by some as a form of confirmation that the Russian claim to have killed Abu Bakr al-Baghdadi may have basis in fact.

The leaning minaret  AFP

The mosque [...] carries symbolic weight in Iraq and the greater Middle East. The Great Mosque of al-Nuri also carries symbolic weight for the terror group: It’s where ISIS founder Abu Bakr al-Baghdadi gave his first speech as caliph in 2014, days after the terror group declared its caliphate in Iraq and Syria. ISIS has held the city as its de-facto capital in Iraq since then, using it as a central hub for commerce and illegal oil sales, and stealing stores of weapons and cash from the Iraqi forces who had abandoned the city. [...] But ISIS, over their newswire Amaq, contradicted reports from Iraqi forces, claiming instead that the U.S. was behind the mosque’s destruction. The U.S.-led coalition forcefully contested that claim, saying in a statement that the “responsibility for this devastation is laid firmly at the doorstep of ISIS.” The coalition added that it had confirmed through drone surveillance that the mosque was destroyed. An investigation is underway. Taking the mosque back from ISIS militants, who have waged a long and bloody campaign to hold the city since Iraqi forces began their offensive eight months ago, would have been a symbolic victory for U.S. and Iraqi coalition forces.

It is believed that ISIL is still holding up to 100,000 civilians in Mosul, using them as human shields during combat. The taking of the western part of Mosul, with its winding roads and small buildings, has proved more difficult than the eastern part which fell five months ago.

The Old City in West Mosul has been the site of the deadliest fighting in the course of the 8-month-long offensive. The Iraqi army believes there to be only 300 Iraqi fighters left in Mosul; there were 6,000 at the start of the offensive, according to Reuters.

BBC News Battle for Mosul: IS 'blows up' al-Nuri mosque 22nd June 2017. 

UPDATE
I am not fully convinced that the much-published footage of the controlled explosions that reportedly depict this tower being felled are authentic, at least one version in the web seems to have been manipulated. This film (disseminated by 'Palmyra Pioneer) shows the damage on the ground and you can see the building was blown flat by explosions from inside and there are no traces whatsoever of craters.  An air-conditioning fan is represented as a bomb fragment.