A man and woman leave a reception center at the Kuala Lumpur International Airport set up for families and friends after an airliner went missing on March 8, 2014. The Malaysian Airlines flight was carrying 227 passengers and 12 crew members. (Manan Vatsyayana/AFP/Getty Images)

A Malaysian Airlines flight carrying 239 people, including four Americans, went missing over the South China Sea as it flew from Kuala Lumpur to Beijing on Saturday, prompting countries in the region to launch maritime search and rescue operations.

The airline said it was still investigating the incident, and that there had been no distress signal from the plane before it lost contact with air traffic control.

Malaysian authorities denied a Vietnamese report that the plane had been found to have crashed into the sea. But with no sign of the plane 12 hours after it vanished, experts said the plane would not have carried enough fuel to still be flying.

Flight MH370 lost contact with Malaysia air traffic control at 2:40 a.m. (18:40 GMT Friday), two hours after taking off from Kuala Lumpur, the airline said in a statement. It had been due to land at Beijing at 6:30 a.m. (22:30 GMT Friday). On board were 227 passengers and 12 crew, from 14 countries, including 152 Chinese citizens. The four Americans on board included one infant.

“We are extremely worried,” China’s Foreign Minister Wang Yi told reporters in Beijing, according to state media. “The news is very disturbing. We hope everyone is safe.”

(By Laris Karklis/The Washington Post/The Washington Post)

Malaysia and Vietnam were conducting a joint search and rescue operation in the area, Malaysia Airlines Chief Executive Ahmad Jauhari Yahya told a news conference in Kuala Lumpur, according to the Reuters news agency, while China has also sent two rescue ships to the area, state television reported.

The Boeing 777-200ER last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, Jauhari said. The deputy chief of staff of Vietnam’s army, Lt. Gen. Vo Van Tuan said the plane’s radar signal had vanished “one minute before it entered Vietnam’s air traffic control,” in a statement carried by the Associated Press.

Col. Dou Kai of Vietnam’s Navy was reported by Chinese state media as having said the plane had crashed into the sea at the junction of the territorial waters of Malaysia and Vietnam. But the Navy Admiral Ngo Van Phat told Reuters only that the plane “could have” crashed in Malaysian waters, 153 miles off the coast of Vietnam’s Tho Cuo island.

Malaysia’s Transport Minister Hishamuddin Hussein said they were still awaiting information from the Vietnamese side.

“We are doing everything in our power to locate the plane. We are doing everything we can to ensure every possible angle has been addressed,” he told reporters near the Kuala Lumpur International Airport, according to Reuters. “We are looking for accurate information from the Malaysian military. They are waiting for information from the Vietnamese side.”

In Beijing, relatives and friends of those on board were taken by minibus from the airport to a hotel in the city to wait for news.

“The airline hasn’t given us any new information, and didn’t tell us how they will handle this,” said one woman in her 30s, who said her father had been on board the plane, and whose voice was breaking.

Malaysia Airlines cited speculation that the plane might have landed at Nanming in Vietnam. But Boeing China President Marc Allen said on his Sina Weibo account that earlier reports that the plane had been found were “in error.”

“The search continues. Our deepest concerns remain with the families of those on board,” he wrote.

At the hotel in Beijing, frustration mounted among relatives at the lack of news. “I haven’t even met a single person from Malaysia Airlines, just Chinese volunteers,” a 30-year old man who had come to meet his cousin told a throng of reporters. “I just need to confirm one thing: they told me it landed, does that mean a crash? Because I heard a crash from the news, and it drives me crazy.”

A little later, a small group of relatives left the hotel.

“Let us out, let us out,” one man shouted as he left. “We have been here for more than three hours. They are keeping 200 family members in a room and only giving us information released at 10:30 am. This is not fair. They didn’t even give us the full list of passengers on the plane, so we are not staying any more.”

If the plane is found to have crashed, it would mark the second fatal accident involving a 777 since it was introduced into service in 1995. In July 2013, an Asiana Airlines Boeing 777-200ER crash-landed in San Francisco, killing three passengers and injuring more than 180.

The United States could have a role in any investigation as the plane was built there, according to Kelly Nantel , a spokeswoman for the U.S. National Transportation Safety Board. “We won’t know who would lead the investigation until the location is identified,” she told the Associated Press.

Boeing said it was assembling a team to help with the investigation. “It’s too early to make any speculation on the causes of the accident,” said China spokesman Wang Yukui.

Malaysia Airlines said its focus was to work with emergency responders and authorities. “Our thoughts and prayers are with all affected passengers and crew and their family members,” it said in a statement.

Liu Liu, Gu Jinglu and Xu Jing contributed to this report.