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), BBC News reports.

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 demanded allegiance in his first and only public appearance following the declaration days earlier of a "caliphate".

The mosque's destruction has brought widespread condemnation.

Lazare Eloundou Assomo, deputy director of the UN's cultural organisation Unesco, said the "iconic" mosque could have helped foster "reconciliation and peace" in Iraq in future.