Islamic State militants on Wednesday, June 21 blew up the Grand al-Nuri Mosque of Mosul and its famous leaning minaret, Iraq's military said in a statement, as Iraqi forces seeking to expel the group from the city closed in on the site, Reuters reports.

It was from this medieval mosque three years ago that the militants' leader Abu Bakr al-Baghdadi declared a self-styled "caliphate" spanning parts of Syria and Iraq.

''Blowing up the al-Hadba minaret and the al-Nuri mosque amounts to an official acknowledgement of defeat,'' Iraqi Prime Minister Haider al-Abadi said in a brief comment on his website.

The Iraqis called the 150-foot (45-metre) leaning minaret Al-Hadba, or "the hunchback." Baghdadi's black flag had flown over it since June 2014.

Islamic State's Amaq news agency accused American aircraft of destroying the mosque, a claim swiftly denied by the U.S.-led coalition fighting the militant group.

"We did not strike in that area," coalition spokesman U.S. Air Force Colonel John Dorrian told Reuters by telephone.

"The responsibility of this devastation is laid firmly at the doorstep of ISIS," U.S. Army Major General Joseph Martin, commander of the coalition's ground component, said in a statement, using an acronym for Islamic State.

The media office for Iraq's military distributed a picture taken from the air that appeared to show the mosque and minaret largely flattened and reduced to rubble among the small houses of the Old City, the historic district where the militants are under siege.

A video seen on social media showed the minaret collapsing vertically in a belch of sand and dust, as a woman lamented in the background, "The minaret, the minaret, the minaret."

The mosque was destroyed as Iraq's elite Counter Terrorism Service (CTS) units, which have been battling their way through Mosul's Old City, got within 50 meters (164 feet) of it, according to an Iraqi military statement.