The formal opening of a former Byzantine church in Istanbul as a mosque “insults its character as a UNESCO world cultural heritage monument belonging to humanity,” the Foreign Ministry has said.

Four years after his government had designated it a Muslim house of prayer, on Monday Turkish President Recep Tayyip Erdogan remotely presided over a ceremony for the reopening of the Church of St. Saviour in Chora, known as Kariye in Turkish, as a mosque.

The decision of the Turkish authorities “constitutes a challenge to the international community, as it alters and insults its character as a UNESCO world cultural heritage monument belonging to humanity,” the Foreign Ministry said. “The preservation of the universal character of monuments and the observance of international standards for the protection of religious and cultural heritage is a clear international obligation that binds all states,” it noted.

The church, situated near Istanbul’s ancient city walls, is famed for its elaborate mosaics and frescoes. It dates to the fourth century, although the edifice took on its current form in the 11th-12th centuries. The structure served as a mosque during Ottoman rule before being transformed into a museum in 1945.