(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.

The destruction could amount to a war crime, according to the UN Human Rights Council.

"Such intentional destruction is an attack on the religious and cultural heritage of the Iraqi people -- and the whole world," the UN said Friday. "International humanitarian law clearly prohibits such acts, and perpetrators who target these objects while being aware of their religious and historical character may be held accountable for war crimes."

Before-and-after photographs of the destruction.

It's difficult to overstate the symbolic importance of the Old City mosque, whose landmark 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.

The Great Nur al-Din Mosque Built by the Seljuk ruler Nur al-Din al-Zangi Atabeg

Completed in 1172

Most notable feature: Al Hadba minaret, which leans 253 centimeters (100 inches)

Minaret was given its nickname by the time famous traveler Ibn Battutah, who visited Mosul in the 14th century

Mosque was dismantled and reassembled in 1942, but minaret remained one of the few original elements Sources: UNESCO and the World Monuments Fund

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 Thursday 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."

The United Nations Educational, Scientific and Cultural Organisation's 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."

Bokova also issued a statement deploring the destruction of the ancient structure. "The Al Hadba Minaret and Al Nuree Mosque in Mosul were among the most iconic sites in the city, and stood as a symbol of identity, resilience and belonging.

"When Daesh targeted the mosque and minaret a few month ago, the people of Mosul formed a human chain to protect the site, proving once again that the protection of heritage cannot be delinked from the protection of human lives," she said.

"This new destruction deepens the wounds of a society already affected by an unprecedented humanitarian tragedy, with 3 million internally displaced persons and 6,2 million in need of immediate humanitarian assistance. This calls for immediate and strengthened international mobilization."

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.

Photos: How ISIS ravaged Palmyra's world treasures Photos: How ISIS ravaged Palmyra's world treasures Before: The ruins of the 2,000-year-old Temple of Baalshamin are seen in Palmyra, Syria, in 2007. The ISIS militant group took over the ancient city last year and razed parts of its The ruins of the 2,000-year-old Temple of Baalshamin are seen in Palmyra, Syria, in 2007. The ISIS militant group took over the ancient city last year and razed parts of its World Heritage Site. Syrian government forces recaptured the ancient city from the terror group in March 2016. Click through to see the landmarks before and after ISIS' occupation. Hide Caption 1 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: A Syrian government soldier walks near what's left of the Temple of Baalshamin on Sunday, March 27. Syrian forces retook the city days before, but damage had already been done by ISIS. UNESCO says it plans to evaluate the extent of A Syrian government soldier walks near what's left of the Temple of Baalshamin on Sunday, March 27. Syrian forces retook the city days before, but damage had already been done by ISIS. UNESCO says it plans to evaluate the extent of Palmyra's damage soon. Many of the structures -- which date from the first and second centuries and marry Greco-Roman techniques with local traditions and Persian influences -- remain in place, bolstering hopes that ISIS didn't completely raze the world heritage site. Hide Caption 2 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before: The Arch of Triumph in 2006. Hide Caption 3 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: The Arch of Triumph on March 27. Hide Caption 4 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before: The Temple of Bel in 2008. Hide Caption 5 of 15 Photos: How ISIS ravaged Palmyra's world treasures After: The Temple of Bel on March 27. Hide Caption 6 of 15 Photos: How ISIS ravaged Palmyra's world treasures Destroyed statues are seen inside the damaged Palmyra Museum on March 27. Hide Caption 7 of 15 Photos: How ISIS ravaged Palmyra's world treasures Many statues in the museum had their faces chipped off -- in keeping with strict Sharia interpretations of the depiction of human forms. Hide Caption 8 of 15 Photos: How ISIS ravaged Palmyra's world treasures Before ISIS invaded, authorities took what they could from the museum. But larger items and those fixed to walls had to be left behind. Hide Caption 9 of 15 Photos: How ISIS ravaged Palmyra's world treasures Destroyed statues inside the Palmyra Museum. Hide Caption 10 of 15 Photos: How ISIS ravaged Palmyra's world treasures The Syrian directorate-general of antiquities and monuments was positive that the condition of artifacts meant that they could be restored and their "historic value" returned, according to a translation of an article on the department's website. Hide Caption 11 of 15 Photos: How ISIS ravaged Palmyra's world treasures ISIS took over Palmyra in May. Hide Caption 12 of 15 Photos: How ISIS ravaged Palmyra's world treasures By June, ISIS began destroying historical sites. Hide Caption 13 of 15 Photos: How ISIS ravaged Palmyra's world treasures Damage inside the Palmyra Museum. Hide Caption 14 of 15 Photos: How ISIS ravaged Palmyra's world treasures Palmyra was a caravan oasis when Romans overtook it in the mid-first century. In the centuries that followed, the area "stood at the crossroads of several civilizations" with its art and architecture mixing Greek, Roman and Persian influences, according to UNESCO. Hide Caption 15 of 15

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.