Rough seas and strong winds have delayed the search for Malaysia Airlines Flight MH370 in the Indian Ocean, prolonging the search until August in one of the world's greatest aviation mysteries, investigators said on Wednesday.

The search had been scheduled to be completed by mid-year.

Australian officials, who are leading the investigation, said in a statement that more than 105,000 sq km (40,540 sq miles) had been covered in the 120,000 sq km search zone.

Seas as high as 18 metres and strong winds battered the search vessels and slowed their progress, Australia's Transport Safety Bureau (ATSB) said.

Flight MH370 disappeared in March 2014 with 239 passengers and crew on board shortly after taking off from Kuala Lumpur bound for Beijing.

Investigators believe someone may have deliberately switched off the plane's transponder before diverting it thousands of miles off course over the Indian Ocean.

Australia, China and Malaysia had agreed the search area would not be expanded beyond 120,000 sq km, the ATSB said in a statement.

A piece of the Boeing 777, a wing part known as a flaperon, washed up on the French Indian Ocean island of Reunion in July last year. - Reuters