Turning the former Byzantine cathedral Hagia Sophia into a mosque would be an "insult" to Christians, the Greek government has said. 

Turkish President Recep Tayyip Erdogan said in a TV interview on Sunday that the UNESCO World Heritage Site in Istanbul could revert back to being a mosque. 

Hagia Sophia was a Greek Orthodox cathedral at the time of the Ottomon conquest of Constantinople, as Istanbul was called then, in 1453, after which it was turned into a mosque. 

In 1935, Turkey's founder Mustafa Kemal Atatürk turned the landmark building into a secular museum.

In an interview with interview with the Turkish broadcaster TGRT on Sunday, Mr Erdogan said it was "not a strange proposal" to suggest turning Hagia Sophia back into a mosque. 

"As you know, the mosque was converted to a museum in 1935, as a reflection of the (Republican People's Party) CHP mentality. We may as well take a step and change that," he was quoted as saying by the Daily Sabah newspaper. 

Greek Foreign Minister George Katrougalos expressed anger at the Turkish president's comments.

"It is not only a great temple of Christendom — the largest for many centuries — it also belongs to humanity. It has been recognized by UNESCO as part of our global cultural heritage," he said. 

"So any questioning of this status is not just an insult to the sentiments of Christians, it is an insult to the international community and international law."