Angel's face uncovered at Istanbul's Haghia Sophia

Restoration workers have uncovered a well-preserved, long-hidden mosaic face of an angel at the former Byzantine cathedral of Haghia Sophia in Istanbul, an official said Friday.

The seraphim figure - one of two located on the side of a dome - had been covered up along with the building's other Christian mosaics shortly after Constantinople - the former name for Istanbul - fell to the Ottomans in 1453 and the cathedral was turned into a mosque.

The mosaics were plastered over according to Muslim custom that prohibits the representation of humans.

Some of the mosaics were revealed when the domed complex was turned into a museum in 1935, but the seraphim had largely remained covered, Ahmet Emre Bilgili, who heads culture and tourism affairs in Istanbul, told The Associated Press.

Two Swiss architects saw the two seraphim during restoration work ordered by the Sultan in the mid-19th century but the figures were covered up again, Bilgili said.

"It is the first time that the angel is being revealed," he said, adding that the figure had been covered with metal and plaster. "It is very well preserved."

Experts would now work to uncover the second seraphim, which was also plastered over and covered by metal, Bilgili said.

The newly uncovered image was hidden behind scaffolding and is not currently visible to visitors.

Haghia Sophia, also called the Church of Holy Wisdom, was built in 537 B.C. and remained a symbol of Byzantine grandeur until Istanbul was conquered by Muslim armies.

The structure was then turned into a mosque - minarets were added and crosses and other Christian symbols were defaced. It became one of the most renowned mosques of the expanding Ottoman Empire.

The site was later converted to a museum under the secular reforms of modern Turkey's founder, Mustafa Kemal Ataturk, attracting thousands of visitors each year.

President Barack Obama toured Haghia Sophia when he visited Turkey in April, as did former U.S. President Bill Clinton in 1999. Pope Benedict XVI also strolled the site in 2006 of his pilgrimage of landmarks of Christianity's ancient roots in Turkey.