Vietnamese air force planes have spotted two large oil slicks that authorities suspect are from a Malaysian jetliner that went missing early on Saturday.
A Vietnamese government statement said the slicks were spotted off the southern tip of Vietnam.
They were each between six miles and nine miles long, officials said.
The statement said the slicks were consistent with the kinds that would be left by fuel from a crashed jetliner.
The Malaysia Airlines Boeing 777 disappeared from radar screens with 239 people on board en route to Beijing.
After the oil slick was spotted, the air search was suspended for the night, while the sea search was ongoing, Malaysia Airlines said.
The plane was carrying 227 passengers, including two infants, and 12 crew members, the airline said.
It said there were 152 passengers from China, 38 from Malaysia, seven from Indonesia, six from Australia, five from India, three from the US, and others from Indonesia, France, New Zealand, Canada, Ukraine, Russia, Italy, Taiwan, the Netherlands and Austria.
Malaysia Airlines CEO Ahmad Jauhari Yahya said there was no indication that the pilots had sent a distress signal, suggesting that whatever happened to the plane occurred quickly.
At Beijing’s airport, authorities posted a notice asking relatives and friends of passengers to gather at a nearby hotel to wait for further information, and provided a shuttle bus service.
Relatives and friends of passengers were escorted into a private area at the hotel, but reporters were kept away. A man in a gray hooded sweatshirt later stormed out complaining about a lack of information. The man, who said he was a Beijing resident but declined to give his name, said he was anxious because his mother was on board the flight with a group of 10 tourists.
“We have been waiting for hours and there is still no verification,” he said.
The plane was last detected on radar at 1.30am (5.30pm Irish time on Friday) around where the South China Sea meets the Gulf of Thailand, authorities in Malaysia and Vietnam said.
Lai Xuan Thanh, director of Vietnam’s civil aviation authority, said air traffic officials in the country never made contact with the plane.
The plane “lost all contact and radar signal one minute before it entered Vietnam’s air traffic control,” Lt Gen Vo Van Tuan, deputy chief of staff of the Vietnamese army, said in a statement.
Malaysian Prime Minister Najib Razak said that Malaysia had dispatched 15 planes and nine ships to the area, and that the US Navy was sending some planes as well. Singapore, China and Vietnam also were sending aircraft.
In Kuala Lumpur, family members gathered at the airport.
“Our team is currently calling the next of kin of passengers and crew. Focus of the airline is to work with the emergency responders and authorities and mobilize its full support,” said Mr Yahya.
“Our thoughts and prayers are with all affected passengers and crew and their family members.”
Fuad Sharuji, Malaysia Airlines’ vice president of operations control, told CNN that the plane was flying at an altitude of 35,000ft (10,670m) when it disappeared and that the pilots had reported no problem with the aircraft.
Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed in San Francisco in July 2013, killing three passengers, all teenagers from China.