Flight MH370 vanished at 02:40 local time Saturday after leaving Kuala Lumpur for Beijing.
The aerial search has been halted for the night but sea operations continue.
No wreckage has been reported by the airline, but Vietnamese planes reported seeing oil slicks in the sea.
The Vietnamese government said two slicks, about 9 miles long, were consistent with those that could be left by an airliner and had been detected off southern Vietnam.
However, there is no confirmation the slicks relate to the missing plane.
Distraught relatives and loved ones of those aboard are being given assistance at the airports.
“We are doing everything in our power to locate the plane,” Malaysian Transport Minister Hishammuddin Hussein told reporters in Kuala Lumpur.
Malaysia Airlines chief executive Ahmad Jauhari Yahya said the focus was on helping the families of those missing. He said that 80% of the families had been contacted.
The plane reportedly went off the radar south of Vietnam.
Its last known location was off the Ca Mau peninsula although the exact position was not clear.
The Boeing B777-200 aircraft was carrying 227 passengers, including two children, and 12 crew members.
Malaysia’s military said a second wave of helicopters and ships had been dispatched after an initial search revealed nothing. The US has agreed to help with its aircraft too, Malaysian PM Najb Razak said.
Territorial disputes over the South China Sea were set aside temporarily as China dispatched two maritime rescue ships and the Philippines deployed three air force planes and three navy patrol ships.
Singapore is also involved, while Vietnam sent aircraft and ships and asked fishermen in the area to report any suspected sign of the missing plane.
“In times of emergencies like this, we have to show unity of efforts that transcends boundaries and issues,” said Lt. Gen. Roy Deveraturda, commander of the Philippine military’s Western Command.
The passengers were of 14 different nationalities. Among them were 152 Chinese nationals, 38 Malaysians, 12 people from Indonesia and six from Australia.
The pilot was Capt. Zaharie Ahmad Shah, 53, who joined Malaysia Airlines in 1981.
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