Hunan province in central China is offering a reward to anyone who can decode the inscription on the back of six ancient gold coins.

The Cultural Relics Bureau of Jinshi city has offered 10,000 yuan (1,500 U.S. dollars) to anyone who can explain the mystery of the coins, housed in the city's museum.

The coins are believed to have been manufactured during the Delhi Sultanate period, sometime in the middle of China's Yuan Dynasty (1271-1358), said the bureau director Peng Jia .

In the 1960s, a small white glazed pot containing six gold foreign coins was discovered at a farm. Each coin is as big as China's one yuan coin currently in circulation.

Since they were sent to the museum in the 1980s, archaeologists have been puzzled. The coins are classified as first-level national cultural relics.

The inscription on the front, in a rare type of Arabic, is the name of a King, said Peng, "but the information on the back is difficult to decode. I have consulted Chinese and foreign experts, but to no avail. I hope the answer will be revealed one day."