The American amateur investigator who found suspected debris from missing Malaysian Airlines flight MH370 told AFP on Thursday that experts must be "cautious" about identifying the piece washed up off Mozambique.

Blaine Gibson, a lawyer from Seattle who has dedicated himself to solving the mystery of the plane's disappearance, explained how he discovered the fragment on a sandbank near the tourist island of Benguerra.

"I was just travelling as a tourist but I have a personal interest to look for debris and find people who may know something (about the fate of MH370)," he told AFP before flying from Maputo to Malaysia.

"It is now in very good hands, and Mr de Abreu (the head of Mozambique's Civil Aviation Institute) is absolutely right to be cautious because there are three planes that crashed in the area.

"We don't know what it is, which plane it is from. It needs to go to Australia to be inspected. I'm very happy I made this discovery, that it happened."

Gibson said he did not think there was any more debris where he found the piece earlier this week.

He told CNN that he was fascinated by the MH370 case, and had paid for his own travel to Malaysia and the Maldives to gather information.

"I've been very involved in the search for Malaysia 370, just out of personal interest... not in a for-profit way or journalistic way," he told the station.

"I went for the one-year commemoration in Kuala Lumpur and met some of the family members and families and it inspired me to keep on looking."

He also told US media that he had chartered a boat to reach the remote sandbank where ocean debris was known to wash up.

Joao de Abreu, the head of Mozambique's civil aviation institute, displayed the fragment in Maputo on Thursday.

"It's very difficult for any crash investigator to confirm which type of plane that piece belongs to," de Abreu said.

The words "No Step" were printed along one side of the flat grey piece.

Last July, a wing fragment was found washed ashore on the Indian Ocean island of Reunion and later confirmed to be from MH370, a Boeing 777.

It is about 1,300 miles (2,100 kilometres) between the two sites.

Malaysian Transport Minister Liow Tiong Lai has said there was a "high probability" that the debris in Mozambique was from a Boeing 777.