A Turkish court said on Friday it annulled a 1934 government decree turning Istanbul's Hagia Sophia into a museum, ruling it was unlawful, paving the way for the building's conversion back into mosque despite international warnings against such a move.

Turkish President Tayyip Erdogan had proposed restoring the mosque, a focal point of both the Christian Byzantine and Muslim Ottoman empires and now one of the most visited monuments in Turkey.

The UN's cultural agency UNESCO warned Turkey earlier on Friday against converting the Byzantine-era Hagia Sophia museum into a mosque, urging dialogue before any decision is taken.

The Hagia Sophia, first a cathedral then a mosque after the conquest of Istanbul, is inscribed as a UNESCO World Heritage site as part of an area designated as "Historic Areas of Istanbul."

"This inscription entails a number of commitments and legal obligations," a UNESCO spokeswoman said.

The sixth-century edifice whose stunning architecture is a magnet for tourists worldwide has been a museum since 1935, open to believers of all faiths.

Hagia Sophia was first constructed as a cathedral in the Christian Byzantine Empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453.

Turning it into a museum was a key reform of the post-Ottoman authorities under the modern republic's founder Mustafa Kemal Ataturk.

But calls for it to serve again as a mosque have sparked anger among Christians and tensions between the historic foes and uneasy North Atlantic Treaty Organization (NATO) allies Turkey and Greece.

The ruling was made by the Council of State, Turkey's top administrative court.

(With input from Reuters and AFP)