Two-thirds of the missing plane's passengers were from China, while others were from elsewhere in Asia, North America and Europe.
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 and possibly catastrophically.
Asked whether terrorism was suspected, Malaysian Prime Minister Najib Razak said, "We are looking at all possibilities, but it is too early to make any conclusive remarks."
Foreign ministry officials in Italy and Austria said the names of two nationals from those countries listed on the flight's manifest matched passports reported stolen in Thailand.
Italy's Foreign Ministry said the Italian man who was listed as being a passenger, Luigi Maraldi, was traveling in Thailand and was not aboard the plane. It said he reported his passport stolen last August.
Austria's Foreign Ministry confirmed that a name listed on the manifest matched an Austrian passport reported stolen two years ago in Thailand. It said the Austrian was not on the plane, but would not confirm the person's identity.
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. A woman wept aboard the bus while saying on a mobile phone, "They want us to go to the hotel. It cannot be good."
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 1:30 a.m. (1730 GMT 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.
The South China Sea is a tense region with competing territorial claims that have led to several low-level conflicts, particularly between China and the Philippines. That antipathy briefly faded Saturday as China, the Philippines, Vietnam, Singapore and Malaysia all sent ships and planes to the region.
Najib said Malaysia had dispatched 15 planes and nine ships to the area. The U.S. Navy was sending a warship and a surveillance plane, while Singapore said it would send a submarine and a plane. China and Vietnam also were sending aircraft to help in the search.
It's not uncommon for it to take several days to find the wreckage of aircraft floating on the ocean. Locating and then recovering the flight data recorders, vital to any investigation, can take months or even years.