Updated Mar 08, 2014 10:50am EST

Military aircraft involved in search and rescue operations to locate a missing Malaysia Airlines plane have spotted two large oil slicks off Southern Vietnam that they believe to be jet fuel.

A statement from the Vietnamese government described the oil slicks as six and nine miles long.

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Flight MH370 en route to Beijing from Kuala Lumpur disappeared from radar screens with 239 people on board about an hour after take-off.

The Boeing 777-200 aircraft left Kuala Lumpur at 12:21am local time and was due to land in Beijing at 6:30am Saturday.

The plane's radar signal had vanished "one minute before it entered Vietnam's air traffic control", said Deputy Chief of Staff Lt. General Vo Van Tuan of the Vietnamese Army.

More than 12 hours after it went missing, a multi-nation search and rescue team said they still had found no sign of the  aircraft.  There were no distress calls from the aircraft, said airline CEO Ahmad Jauhari Yahya.

By mid-afternoon of Saturday, Malaysian authorities were denying an earlier Vietnamese report that the plane had been found to have crashed into the sea off Vietnam.

"We are extremely worried of the Vietnamese report that the plane had crashed," Chinese Foreign Minister Wang Yi told reporters in Beijing. "The news is very disturbing. We hope everyone on the plane is safe."

"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," Malaysia's Transport Minister Hishamuddin Hussein told reporters in Kuala Lumpur.

Malaysia Airlines said the 227 passengers were of 14 different nationalities, including 154 citizens of China and Taiwan, 38 Malaysians, 12 Indonesians, six Australians and three Americans. The 12 crew members were all Malaysians. A Chinese infant and an American infant were among those on board the flight, the airline said.

Several Chinese artists returning home from an exhibit are said to be on board the missing plane.  The airline company did not release the flight manifest but said it had contacted nearly all the passengers' nearest of kin. 

Relatives of the passengers were brought to a hotel in Beijing to await word about the status of the missing plane.  They were shielded from the media but as grief and frustration mounted during the long wait, several family members had gone out of the hotel to complain to reporters about the lack of information reaching them.

China has sent two maritime rescue ships to the South China Sea for search and rescue efforts. Malaysia, Vietnam, Singapore and the Philippines also sent their air and naval forces to help scour waters and land on the flight path of the missing plane.