Mosul/Erbil, Iraq (Reuters): Daesh (IS) terrorists on Wednesday destroyed Mosul’s historic the Grand al-Nuri Mosque 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.

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

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

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 terrorist group.

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

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 responsibility of this devastation is laid firmly at the doorstep of ISIS."

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."

According to an Iraqi military statement, 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.

An Iraqi military spokesman gave the timing of the explosion as 9:35 p.m (1835 GMT).

U.S. Major General Martin said, "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."

Iraqi forces said earlier on Wednesday that they had started a push toward the mosque.

Lieutenant General Abdul Ghani al-Assadi, senior CTS commander in Mosul, said in a video posted over a messaging app, ''This will not prevent us from removing them, no, killing them not removing them, inside the Old City.''

Earlier on Tuesday, the forces had encircled the jihadist group's stronghold in the Old City, the last district under Islamic State control in Mosul.