UNESCO said on Friday its World Heritage Committee would review Hagia Sophia's status after Turkish President Tayyip Erdogan declared the ancient monument in Istanbul a mosque again.
Erdogan made the announcement shortly after a top court ruled that the sixth-century building's conversion to a museum in 1934 by Mustafa Kemal Ataturk, founder of the modern secular Turkish republic, was illegal.
UNESCO said that decision raised questions about the impact on its universal value as a site of importance transcending borders and generations, which is necessary to be included on its coveted list of World Heritage sites.
Countries must notify the United Nation's cultural body of any changes in the status of a site, triggering a review by its World Heritage Committee if need be, it said in a statement.
"It is regrettable that the Turkish decision was not the subject of dialogue nor notification beforehand," the United Nation's cultural body said in a statement.
"UNESCO calls on the Turkish authorities to open a dialogue without delay in order to avoid a step back from the universal value of this exceptional heritage whose preservation will be reviewed by the World Heritage Committee in its next session," it said.
The World Heritage site was at the centre of both the Christian Byzantine and Muslim Ottoman empires and is today one of Turkey's most visited monuments, revered by Christians and Muslims alike.
The United States, Russia, Greece and Christian church leaders had urged Turkey to maintain Hagia Sophia's status as a museum.