Malaysia's Acting Transport Minister Hishammuddin Hussein said the focus of the hunt for Malaysia Airlines MH370 is still on narrowing the search corridors.

"I can confirm that we have received some radar data, but we are not at liberty to release information from other countries. I appeal to all our partners to continue volunteering any and all information that could help with the investigation and the search for MH370," he told a regular press briefing on Wednesday.

Malaysia has written to all 26 countries formally requesting cooperation, he said.

"A number of assets which have already been committed are awaiting diplomatic clearance to begin operations. Once we receive formal clearance, we can then speed up the deployment of assets along the search corridors".

The search now covers 2.24 million square nautical miles, or four-fifths the size of the United States, and stretches from the border of Kazakhstan and Turkmenistan in Central Asia to southern Indian Ocean.

The search for MH370, which disappeared on March 8 while travelling from Kuala Lumpur to Beijing, entered day 12 on Wednesday.

It is the longest time any civilian aircraft of this size has been untraceable. In 2007, it took 10 days to find an Adam Air plane which crashed en route from Surabaya to Manado.

With seemingly few clues and an "enormous" search zone, Malaysia said on Tuesday that "all efforts are now to reduce the area of concentration".

This is being done by reviewing available satellite data and asking countries with satellite capabilities for help. Malaysia is also asking countries the plane may have flown over to check their radar data.