Portuguese authorities arrested two Australian men over attempts to smuggle six pieces of rhinoceros horns valued at $538,000.

The two were caught while about to board their flight to Ireland on Friday, AFP reported. They are the suspects over the loss in April 2011 of two rhino horns dating back to the 18th century allegedly taken from a history museum in Coimbra. The items were found inside their luggage.

Police inspector Rui Almeida told AP that the two men, a father and son team, are likely members of an international syndicate involved in smuggling of rhino horns to China based on an observed rising incidents of rhino horn thefts across the continent.

Among the recent victims was the Museum of Natural History at the University of Coimbra, although the Portuguese police said the items the two took likely came from another museum in the same city.

The horns are stolen for its purported aphrodisiac and medicinal qualities. Demand for the rhino horn is particularly high in Asia.

Portuguese law prevents media identification of the two Australians who were released on bail because smuggling carries only a maximum prison term of five years. The Australian Department of Foreign Affairs and Trade offered assistance to the two Australian nationals, aged 63 and 31.