Mr Najib praised the search effort, saying the co-operation amid “great tragedy” had “given us all heart”.
The Malaysia Airlines plane disappeared on 8 March en route from Kuala Lumpur to Beijing. It was carrying 239 people.
Planes and ships have been scouring the southern Indian Ocean, where the jet is believed to have crashed.
The focus of the search is a 221,000 sq km (85,000 sq mile) area 1,500km (932 miles) west of Perth.
But so far not a single piece of debris from the Boeing 777 has been found.