Daish militants on Wednesday blew up the Grand al-Nuri Mosque of Mosul and its famous leaning minaret, an Iraqi military statement said, 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 militants' leader Abu Bakr al-Baghdadi declared a self-styled "caliphate" spanning parts of Syria and Iraq.

Daish's Amaq news agency accused American aircraft of destroying the mosque, a claim swiftly denied by the U.S.-led international coalition fighting the hardline Sunni group.

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

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

The Iraqi military's media office distributed a picture taken from the air that appeared to show the mosque and minaret flattened in the middle of the small houses of the Old City, the historic district where the militants are besieged.

"The Daish terror gangs committed another historical crime by blowing up the al-Nuri mosque and its historical al-Hadba minaret," the Iraqi military statement said.

The Iraqis lovingly call the minaret Al-Hadba, or "the hunchback."

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

The explosions happened as Iraq's elite Counter Terrorism Service units, which have been battling their way through Mosul's Old City, got to within 50 meters (164 feet) of the mosque, the Iraqi military statement said.

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

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

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

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

Al-Baghdadi proclaimed himself "caliph" - or ruler of all Muslims - from the mosque's pulpit on July 4, 2014, after the insurgents overran vast swathes of Iraq and Syria.

His black flag had been flying over its 150-foot (45-metre)leaning minaret since June 2014.

Baghdadi's speech from the mosque was also the first time he revealed himself to the world, and the footage broadcast then is to this day the only video recording of him as "caliph."

MINARET WAS VULNERABLE

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.

"The battle for the liberation of Mosul is not yet complete, and we remain focused on supporting the Iraqi Security Forces with that objective in mind," said Martin.

The fall of Mosul would, in effect, mark the end of the Iraqi half of the "caliphate" even though Daish would continue to control territory west and south of the city, the largest they held sway over in both Iraq and Syria.

Baghdadi has left the fighting in Mosul to local commanders and is believed to be hiding in the border area between Iraq and Syria, according to U.S. and Iraqi military sources.

The mosque is named after Nuruddin al‑Zanki, a noble who fought the early crusaders from a fiefdom that covered territory in modern-day Turkey, Syria and Iraq. The mosque was built in 1172-73, shortly before his death, and housed an Islamic school.

By the time renowned medieval traveller Ibn Battuta visited two centuries later, the minaret was already leaning. Its tilt gave the landmark its popular name: the hunchback.

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

Nabeel Nouriddin, a historian and archaeologist specialising in Mosul and its Nineveh region, said the minaret has not been renovated since 1970, making it particularly vulnerable to blasts even if it was not directly hit.

The Mosque's destruction occurred during the holiest period of the Islamic holy month of Ramadan, its final 10 days. The night of Laylat al-Qadr falls during this period, marking when Muslims believe the Quran was revealed to prophet Mohammed.