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(CNN) — The United States and Iraq said ISIS blew up a historic mosque in Mosul that was the ideological heart of the terror group and the birthplace of its self-declared caliphate.

ISIS, through its news agency, said US warplanes were responsible for the loss late Wednesday of the Great Mosque of al-Nuri and its leaning minaret.

US officials told CNN the ISIS claim was “1,000% false.”

Iraq Prime Minister Haider al-Abadi said the ISIS act amounts to “an official announcement of their defeat.” His military commanders said militants blew the mosque up after troops closed in.

It’s difficult to overstate the symbolism of the Old City mosque, whose leaning minaret rose over the city for more than 800 years.

It was during Friday prayers here on July 4, 2014, that ISIS leader Abu Bakr al-Baghdadi declared the founding of a new caliphate and called on fellow Sunnis to carry out a holy war. This was the first and last time the leader of the terrorist group spoke publicly to his followers. The mosque’s imam had been executed about a month earlier for refusing to join ISIS, according to the United Nations.

Baghdadi’s declaration effectively broke down borders between Syria and Iraq, creating a magnet for foreign fighters wanting to join ISIS’ cause. For years, the militant group’s black and white flag fluttered from the minaret, a symbol of ISIS’ control.

But in recent months western Mosul has witnessed fierce fighting between ISIS militants and coalition forces who are determined to liberate what was the country’s second-largest city.

The Islamic complex has been very much on the mind of the Iraqi forces, who believed taking control of the mosque would be a highly symbolic victory. Federal police earlier this year said they looked forward to praying in al-Nuri — but the resistance continued.

Now the centuries-old mosque complex lies largely in ruins.

Several US officials have told CNN in recent days that US and coalition officials had been observing the mosque in recent days and saw fighters and explosives at the site. The Iraqi military said “ISIS terrorist gangs” blew up the mosque as Iraqi forces were approaching.

Destruction came as troops moved in

The situation in Mosul is desperate. As ISIS is being squeezed into even smaller territory — a handful of neighborhoods and Mosul’s Old City — the civilians held hostage are running out of food. Residents said ISIS fighters have massacred people, young and old, trying to flee.

The UN children’s agency, Unicef, said Wednesday that children in west Mosul “are being deliberately targeted and killed to punish families and deter them from fleeing the violence.” In less than two months, at least 23 children have been killed and 123 injured in just that part of the city, it said.

About 100,000 civilians remain in the complex battlefield.

Lt. Gen. Abdul Ghani al-Assadi, commander of Iraqi counterterrorism, said Wednesday that he was shocked by the destruction of the mosque but it was not the first time ISIS had targeted Iraq’s cultural heritage.

“ISIS had prepared to blow it up, they were only waiting to see how far our forces can reach,” he said. “We are no more going to drive them out of the Old City, we are going to kill all of them in the coming days.”

American military officials deplored the destruction of the mosque.

“As our Iraqi Security Force partners closed in on the al-Nuri mosque, ISIS destroyed one of Mosul and Iraq’s great treasures,” said US Maj. Gen. Joseph Martin.

Lt. Gen. Stephen Townsend, the top US commander in the fight against ISIS in Syria and Iraq, said: “I was just in Mosul Wednesday afternoon and close enough to see the mosque and its famous leaning minaret. Little did I know it was for the last time. This is just another example that ISIS is a cruel, heartless and godless ideology that cannot be permitted to exist in this world.”

ISIS’ obliteration of history

The Special Representative of the UN Secretary-General for Iraq, Jan Kubis, said Thursday that ISIS fighters’ destruction of the mosque was a “barbaric act” which “shows their desperation and signals their end.”

UNESCO Director-General Irina Bokova tweeted that the loss of the mosque was “a cultural (and) human tragedy,” adding: “We must protect heritage to protect people.”

ISIS has rampaged through numerous cultural heritage sites in Iraq and Syria in the past three years. The extremist group is part of a puritanical strain of Islam that considers all religious shrines idolatrous.

ISIS fighters earlier this year destroyed part of the historic Roman amphitheater in the ancient Syrian city of Palmyra, according to reports. That followed earlier destruction in the archaeological site.

Fighters also destroyed Iraq’s ancient Assyrian city of Nimrud in March 2015.

Earlier that year, militants shoved stone statues off pedestals in the Mosul Museum and took sledgehammers to them and other artifacts. In July 2014, extremists in Mosul also destroyed what was believed to be the tomb of Jonah, a key figure in Christianity, Judaism and Islam.

Iraq’s Ministry of Tourism and Antiquities said in 2015 that it had received reports the ancient Assyrian capital of Khorsabad had been destroyed.