Chinese air force personnel aboard a surveillance plane take part in the search on Wednesday. [Photo/Xinhua]

Malaysia's ambassador to China, Iskandar Sarudin, met family members at the Lido Hotel in Beijing on Wednesday.

He said he will pass their demand to the Malaysian government that the country's military publicize information about its radar tracking of the aircraft before it vanished.

Malaysian authorities said earlier that air defense radar picked up traces of what might have been the plane turning back and flying until it reached the Strait of Malacca, a busy shipping lane west of the nation about 400 km from the plane's last known coordinates.

Malaysia's Acting Transport Minister, Dato Seri Hishammuddin Hussein, said on Wednesday the search has been expanded to two areas, Malacca and the South China Sea.

"We examined our military radar records for the new search area and discovered the possibility that MH370 had passed over the Straits of Malacca. We then dispatched ships and aircraft to search the new area," he said.

"Even if there is only a slight possibility, we decided it was imperative to extend the search to this area," he added.

The last radio transmission from the cockpit of the plane was "all right, good night", it emerged in Beijing on Wednesday.

One of the pilots said these words as the flight passed from Malaysian to Vietnamese airspace, according to Singapore's Straits Times newspaper, quoting Iskandar Sarudin, Malaysia's ambassador to China.

Malaysia's Department of Civil Aviation chief Azharuddin Abdul Rahman later confirmed to Agence France-Presse that those were the last words from the cockpit.

Authorities have not ruled out any possible cause for the plane's disappearance, including mechanical failure, pilot error, sabotage and terrorism.

Hou Liqiang, He Liu and agencies contributed to this story.