Malaysia, Vietnam search for airliner carrying 239
Vietnamese authorities said contact with Flight MH370 was lost near the country's airspace, but its exact location remained a mystery 12 hours after it went missing.
The airline has said the plane, on an overnight flight from Kuala Lumpur to Beijing, relayed no distress signal or other indications of trouble.
"The plane lost contact near Ca Mau province airspace as it was preparing to transfer to Ho Chi Minh City air traffic control," a statement posted on the official Vietnamese government website said.
The plane's signal was never transferred to Ho Chi Minh air traffic control, it added.
Ca Mau province is in southernmost Vietnam.
The ministry launched a rescue effort to find the plane, working in coordination with Malaysian and Chinese officials, the statement added.
Malaysian authorities dispatched a plane, two helicopters and four vessels to search seas off its east coast in the South China Sea, said Faridah Shuib, a spokeswoman for the Malaysian Maritime Enforcement Agency.
The Philippines said it was sending three navy patrol boats and a surveillance plane to help efforts.
- Contact lost two hours after take-off -
Contact was lost at 2:40 am Malaysian time (1840 GMT Friday), about two hours after take-off from Kuala Lumpur International Airport, the carrier's CEO Ahmad Jauhari Yahya said.
"Ladies and gentlemen, we are deeply saddened this morning with the news on MH370," he told a press conference in Malaysia.
"Our focus now is to work with emergency responders and authorities, and mobilise full support," he added.
"And our thoughts and prayers are with all affected passengers and crew, and their family members."
The airline and Malaysian authorities were liaising with Vietnamese officials, he added.
The Boeing 777-200 was carrying 227 passengers and 12 crew members, Ahmad Jauhari said. However, the nationality breakdown he gave added up to 228 passengers.
They included 153 Chinese nationals plus one infant, 38 Malaysians, and 12 Indonesians.
Seven Australians also were aboard, three French nationals, four from the United States including one infant, plus passengers from several other countries.
Malaysia Airlines has a good safety record. Its worst-ever crash occurred in 1977, when 93 passengers and 7 crew perished in a hijacking and subsequent crash in southern Malaysia.
The pilot of MH370 is Capt. Zaharie Ahmad Shah, 53, who has flown for the airline since 1981, the carrier said.
Its first officer Fariq Abdul Hamid, 27, joined the airline in 2007.
The plane is more than 11 years old.
The flight path passes over the South China Sea and the Indochinese peninsula before entering southern Chinese airspace.
"This news has made us all very worried," Chinese Foreign Minister Wang Yi said in Beijing.
"We hope every one of the passengers is safe. We are doing all we can to get more details."
Chinese and Thai authorities have said the plane did not enter their airspace.
- 'Good safety record' -
Screens at Beijing's airport indicated at first that the flight was "delayed", but later updated its status to "cancelled".
An accident would be a huge blow for the carrier, which has bled money for years as its struggles to fend off competition from rivals such as fast-growing AirAsia.
"Malaysia Airlines overall has a good track record" on safety, said Shukor Yusof, aviation analyst with Standard & Poor's.
"This incident comes at a most unfortunate time for the airline which is undergoing a transformation to return to profit."
The flag carrier recorded its fourth straight quarterly loss during the final three months of 2013, and warned of a "challenging" year ahead due to intense competition.
The carrier admitted in 2012 it was in "crisis", forcing it to implement a cost-cutting campaign centred on slashing routes and other measures.
Analysts have said poor management, government interference, and union resistance to reform of the 66-year-old airline have hampered its ability to respond to intensifying competition in the industry.
The Boeing 777 also has a solid safety record, with only a handful of incidents since its introduction in the mid-1990s.
In July 2013, a Boeing 777-200 operated by South Korea's Asiana Airlines skidded off the runway upon landing at San Francisco's international airport after it clipped a seawall before touching down.
Three people died.
"We're closely monitoring reports on Malaysia flight MH370. Our thoughts are with everyone on board," US-based Boeing said in a statement on its Twitter feed.
The information vacuum regarding the flight touched off a frenzy on social media, which saw an outpouring of concern for passengers.
At Beijing airport, an AFP journalist saw one woman at enter the arrivals zone and break down in tears. She was led away by police.
At Kuala Lumpur International Airport, family members looking sombre and distraught trickled in to a designated waiting area for passenger relatives, escorted by authorities.
"They gave us no information so far," complained one man, who said his niece and her husband were on the flight for a one-week holiday in China.
Reporters were barred from entering the area.