A team of international archeologists has discovered grim relics of slavery some 300 years ago on the site of one of the largest "slave castles" in west Africa, the project director said Friday.

Joe Opala said that Bunce Island, some 20 miles (30 kilometres) upriver from Sierra Leone's capital, Freetown, had a "very strong and clear link" with the north American slave trade.

Christopher DeCorse, head of the Department of Anthropology at Syracuse University in New York added, "Bunce Island was known as one of the chief suppliers of slaves to the rice industry of the then British colonies of South Carolina and Georgia."

"African farmers with rice growing skills were kidnapped from inland areas and brought to the castle and then shipped as human cargo to America", he said.

"We are working to preserve the island's structural resources," he added, after the team reported the discovery of an iron shackle and a spike just outside the castle.

"What made the find important is that it tells us really what happened in the island when slaves were being transported, as the shackles were used to restrict their movements, binding their arms and legs to prevent them from becoming violent," DeCorse said.

DeCorse said the spike would be given to the national museum while the shackle would be taken taken to the United States for conservation treatment and then returned to Sierra Leone.

One notable visitor to Bunce Island was former US secretary of state Colin Powell, who after a visit in 1991 remarked, "I am an American but today, I am something more. I am an African too. I feel my roots here in this continent."