Iraqi Prime Minister Haider al-Abadi says the destruction of an ancient mosque in the city of Mosul is "an official declaration of defeat" by so-called Islamic State (IS).
Iraqi forces say IS blew up the Great Mosque of al-Nuri and its famous leaning minaret as jihadists battled to stop advancing pro-government troops.
IS said American aircraft had destroyed the complex, a claim denied by the US.
Aerial photographs show the complex largely destroyed.
The mosque, which was more than 800 years old, was where in July 2014 IS leader Abu Bakr al-Baghdadi commanded allegiance in his first and only public appearance following IS' declaration days earlier of a "caliphate".
The mosque's destruction has brought widespread condemnation.
A senior US commander in Iraq said IS had destroyed "one of Mosul and Iraq's great treasures".
"This is a crime against the people of Mosul and all of Iraq, and is an example of why this brutal organisation must be annihilated," Maj Gen Joseph Martin said.
The Iraqi commander in charge of the offensive to retake Mosul said troops were within 50m (160ft) of the mosque when IS "committed another historical crime".
Thousands of Iraqi security forces, Kurdish Peshmerga fighters, Sunni Arab tribesmen and Shia militiamen, supported by US-led coalition warplanes and military advisers, are involved in the battle to retake the key city, which was launched on 17 October last year.
The government announced the full "liberation" of eastern Mosul in January but the west of the city has presented a more difficult challenge, with its narrow, winding streets.
The al-Nuri mosque dated back to 1172 and was named after a noble who fought against the early crusaders.
The leaning minaret was nicknamed "the hunchback".
A month after IS forces overran Mosul in June 2014, Baghdadi gave a Friday sermon from the pulpit inside the mosque and spoke about the newly-proclaimed caliphate - a state governed in accordance with Islamic law, or Sharia, by God's deputy on Earth, or caliph.
On Sunday, commanders announced the start of the "final chapter" of the offensive, with Iraq's counter-terrorism service, army and federal police attacking the Old City from all directions.
The army said it believed there were no more than 300 militants left in Mosul, compared with almost 6,000 at the start of the offensive in October.
Earlier this week aircraft dropped leaflets urging civilians to avoid open spaces and to take any opportunity to escape.
The UN has warned that IS may be holding more than 100,000 people in Mosul as human shields.
But on Tuesday Iraqi forces said they were releasing civilians previously trapped under IS control in the Old City.
In statements to state-run Iraqia TV, a military official said: "We have broken the siege of IS and allowed the civilians to leave. This is evidence that IS has lost control."
(Source: BBC)