A team of American explorers on Thursday claimed to have discovered silver treasure from the infamous 17th-century Scottish pirate William Kidd in a shipwreck off the coast of Madagascar.

Marine archaeologist Barry Clifford told reporters he had found a 50-kilogramme (110-pound) silver bar in the wreck of Kidd's ship the "Adventure Gallery", close to the small island of Sainte Marie.

Captain Kidd, who was born in Scotland in about 1645, was first employed by British authorities to hunt pirates, but he turned himself into a ruthless criminal of the high seas.

After looting a treasure-laden ship in 1698, he was caught, imprisoned and questioned in front of the British parliament before being executed in Wapping, close to the River Thames in 1701.

The fate of much of his booty, however, has remained a mystery, sparking intrigue and excitement for generations of treasure-hunters.

Clifford, who was filmed by a documentary crew lifting the silver bar off the sea bed, handed it over to Malagasy President Hery Rajaonarimampianina on Sainte Marie.

Soldiers guarded the apparent treasure at the ceremony, which was attended by the US and British ambassadors.

"We discovered 13 ships in the bay," Clifford said. "We've been working on two of them over the last 10 weeks.

"One of them is the Fire Dragon, the other is Captain Kidd's ship, the 'Adventure Galley'."

Independent archaeologist John de Bry, who attended the ceremony, said the shipwreck and silver bar were "irrefutable proof that this is indeed the treasure of the 'Adventure Gallery'."

US marine archaeologist Barry Clifford -- who in 2014 claimed to have located Christopher Columbus' ship 'The Santa Maria' off the coast of Haiti -- is leading the quest to find William Kidd's ship near Madagascar Don Emmert, AFP/File

Robert Yamate, US ambassador to Madagascar, said the discovery was a boost for the country.

"This is a fantastic find that shows the hidden story of Madagascar," he said.

"This is great for tourism... and it is just as important as historical preservation."