Istanbul - A rare oil painting by Islam's last caliph, the whereabouts of which were unknown for decades, fetched 1.6 million lira (S$1 million) at auction on Sunday.

An anonymous collector secured "Women in the Courtyard," by Abdulmecid II in 1899, after a hotly contested round of bidding.

The asking price had been 1.2 million lira.

The painting depicts semi-naked women attended by harem eunuchs around a pool. The work pays homage to the Roman goddess Venus and is a clear nod to 19th-Century Orientalist painters, like France's Jean-Leon Gerome.

"There have been questions about whether the conservative government and new elite would be comfortable with a painting by the caliph that features nudes," Bora Keskiner of Alif Art, the Istanbul auction house which handled the sale, told Reuters.

"The work is an important signifier of how the late Ottoman Turkish elite and royal family were extremely close to European art circles and they themselves practised European art."

Prime Minister Tayyip Erdogan, whose party is rooted in political Islam, frequently evokes the glories of certain Ottoman sultans, who ruled a vast empire from the Balkans to the Persian Gulf for more than four centuries.

The secular Turkish republic was formed in the empire's wake following World War One.

The existence of "Women in the Courtyard" was unknown until 1990, when its image appeared on the cover of an art magazine with little explanation, and it remained a secret in a private collection until its consignment to Alif Art, Keskin said.

Abdulmecid II was crown prince of the Ottoman throne, held by his cousin Mehmed VI, until the sultanate was abolished in 1922 by the founders of the secular Turkish republic.

Mehmed was exiled, and Abdulmecid became caliph, a title through which Ottoman rulers long claimed religious leadership of all Sunni Muslims. Within two years, the caliphate too was elimated and Abdulmecid fled to Paris where he died in 1944.

Besides his religious vocation, Abdulmecid was also one of the great artists of the late Ottoman period. Today, his self-portrait hangs in the Istanbul Museum of Modern Art.