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

On Friday, Turkish president, Tayyip Erdogan, declared the ancient building was open to Muslim prayer after a court ruled that its conversion to a museum back in 1934 was unlawful.

"My thoughts go to Istanbul. I'm thinking about Hagia Sophia. I am very distressed," the Pope said.

Built in the early 6th century, Hagia Sophia was the pinnacle of Byzantine architecture, and the largest church in the world for nearly a thousand years.

After the Ottoman Empire (1290-1922) conquest of Constantinople in 1453, the Hagia Sophia was converted into a mosque, which lasted until the early 20th century.

After the founding of the Republic of Turkey, the building, which witnessed the rise and fall of several empires, reopened as a museum and became a symbol of the blending of East and West cultures and one of Turkey's most popular tourist attractions.

Prior to Friday's ruling, religious services were banned at the site, but Turkey's devout Muslims have long called for that decision to be reversed. 

Throughout his 17-year rule, Erdogan has discussed converting the the sixth-century building back into a mosque as part of a long campaign to bring Islam into mainstream Turkish politics.

The move has been met with condemnation from countries including Greece and Russia, and UNESCO said it "deeply regrets" the decision and called on the Turkish authorities to "open a dialogue without delay."

The World Council of Churches released an open letter calling for Erdogan to reverse his decision.

"By deciding to convert the Hagia Sophia back to a mosque you have reversed that positive sign of Turkey's openness and changed it to a sign of exclusion and division," the World Council of Churches' Interim Secretary General Ioan Sauca said.

"The decision to convert such an emblematic place as Hagia Sophia from a museum back to a mosque will inevitably create uncertainties, suspicions and mistrust, undermining all our efforts to bring people of different faiths together at the table of dialogue and cooperation."

Read more: Turkey turns iconic Hagia Sophia museum into a mosque