Malaysia's prime minister says a Malaysia Airlines jetliner did not make any distress call before it went down in Ukraine, and that the flight route was declared safe by the International Civil Aviation Organisation.
Najib Razak told reporters that Ukrainian authorities believed the Boeing 777, on its way from Amsterdam to Kuala Lumpur, was shot down.
He said Malaysia, however, was unable to verify "the cause of this tragedy but we must, and we will, find out precisely what happened to this flight."
Razak earlier said he was shocked to hear the news about flight MH17.
"I am shocked by reports that an MH plane crashed. We are launching an immediate investigation," Najib said on his Twitter feed. MH is the code for Malaysia Airlines.
This was the second tragedy to hit Malaysia Airlines this year. Its Flight MH370 disappeared on March 8 while en route from Kuala Lumpur to Beijing. It has not been found, but the search has been concentrated in the Indian Ocean west of Australia.
Najib was on his way to the operations centre at Kuala Lumpur International Airport, an official in his office said.
Malaysia Airlines said in a statement it had received notification from Ukrainian air traffic control that it had lost contact with flight MH17 at 2.15pm GMT, 30 km from the Tamak waypoint, approximately 50 km from the Russia-Ukraine border.
The Boeing 777 had left Amsterdam at 12.15pm (Amsterdam time) and was expected to arrive in Kuala Lumpur at 6.10am (2210 GMT) on Friday, the airline said.
The airline said there were 298 passengers and crew on board.