Pope Francis has said he is "very distressed" over Turkey's decision to convert the Byzantine-era monument Hagia Sophia back into a mosque.

"My thoughts go to Istanbul. I'm thinking about Hagia Sophia. I am very distressed," the pope said in the Vatican's first reaction to a decision that has drawn international criticism.

The Vatican newspaper Osservatore Romano had on Saturday carried reaction from different countries about Friday's decision to turn the monument from a museum back into a mosque but without any comment.

A magnet for tourists worldwide, the 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.

Turkish President Recep Tayyip Erdogan, who critics say is chipping away at the Muslim-majority country's secular pillars, announced on Friday that Muslim prayers would begin on 24 July at the UNESCO World Heritage site.

In the past, he has repeatedly called for the building to be renamed as a mosque and in 2018, he recited a verse from the Koran at Hagia Sophia. 

Mr Erdogan's announcement came after a top court cancelled a 1934 cabinet decision under modern Turkey's secularising founder Mustafa Kemal Ataturk to preserve the church-turned-mosque as a museum.