The UN's cultural agency UNESCO said it deeply regretted Turkey's decision to turn the Hagia Sophia in Istanbul from a museum into a mosque, lamenting there had been no prior dialogue on the status of the former Byzantine cathedral.
UNESCO chief Audrey Azoulay "deeply regrets the decision of the Turkish authorities, taken without prior dialogue, to modify the status of the Hagia Sophia," the UN agency said in a statement,
It added that she had expressed her concern to the Turkish ambassador to the body.
The Hagia Sophia was first a cathedral, then made into mosque after the conquest of Istanbul by the Ottomans, but then a secular museum for all in modern Turkey.
It is inscribed as a UNESCO World Heritage site as part of an area of the city designated as "Historic Areas of Istanbul."
UNESCO warned that the move risked harming the universal nature of Hagia Sophia as a place open to all of civilisation, a key aspect of its World Heritage status.
It said that any modification requires prior notification to UNESCO and possibly examination by its World Heritage Committee, which adds – and sometimes removes – sites from the coveted list of UNESCO World Heritage.
"This decision announced today raises the issue of the impact of this change of status on the property's universal value," said UNESCO.
It warned that the "state of conservation" of the Hagia Sophia would be examined by the World Heritage Committee at its next meeting.
"UNESCO calls upon the Turkish authorities to initiate dialogue without delay, in order to prevent any detrimental effect on the universal value of this exceptional heritage," the statement said.