Cambodian Deputy Prime Minister and Foreign Minister Hor Namhong on Wednesday extended his deep sympathy to the families of the passengers on board the missing Malaysian Airlines flight MH370.

The flight disappeared over south Vietnam on March 8.

"I am deeply saddened to learn of the recent missing plane of the flight MH370, flying from Kuala Lumpur to Beijing with 239 people on board," he said in a letter sent to Malaysian Foreign Minister Anifah Aman.

"On this very sad moment, I would like to convey to Your Excellency as well as the bereaved families, my deepest sorrow and profound condolences for this most unfortunate incident," he said.

The airliner, with 239 people including 154 Chinese nationals on board, went missing on its way from Kuala Lumpur to Beijing on March 8, the airlines said, adding that it lost communication and radar signal two hours into the flight over south Vietnam at 1:20 a.m. that day.

Malaysia on Saturday decided to shift the search to one of the two possible corridors -- a northern one from the border of Kazakhstan and Turkmenistan to northern Thailand, and a southern one stretching from Indonesia to the southern Indian Ocean.

On Tuesday, the Cambodian Ministry of Defense sent four Z-9 utility helicopters and two Navy vessels to conduct search operations for the missing jetliner at sea and on land within the country's territory.

Malaysia's acting Transport Minister Hishammuddin Hussein said the number of countries involved in the search and rescue operation had risen to 26 on Monday. Endi