A passenger plane which crashed as it flew over eastern Ukraine with 295 people on board was reportedly shot down.
The Malaysia Airlines plane, which was flying from Amsterdam to Kuala Lumpur, was travelling at an altitude of 33,000 feet (10,000 metres) when it was shot down, Russia's Interfax reported.
An adviser to the Ukrainian interior ministry told the news agency the Boeing 777 was brought down by a Buk ground-to-air missile.
All 280 passengers and 15 crew members who were on the plane are believed to have died, he added.
A spokesman for Malaysia Airlines, still reeling from the loss of flight MH370 in March, confirmed it had lost contact with flight MH17, which took off from Amsterdam's Schipol airport at 12.15pm local time.
The flight disappeared from radar as it flew over Ukrainian airspace, the spokesman said.
A number of videos apparently filmed near the village of Grabovo, Donetsk, where the plane came down, show plumes of thick, black smoke rising high into the air.
TV channel Russia 24 broadcast similar pictures, while a Reuters correspondent at the scene said he could see the wreckage of a burning aircraft and bodies on the ground.
An eyewitness said the plane split in two on impact.
Alexander Borodai, the eastern Ukraine separatist leader, said the aircraft was shot down by Ukrainian government forces – a claim backed by a separatist from Krasnyi Luch, who told Reuters the rebels did not have weapons capable of shooting down a plane at such height.
However, officials in Kiev denied any involvement, with Ukrainian Prime Minister Arseniy Yatsenyuk ordering an immediate investigation into what he described as a "catastrophe".
The Kremlin said Russian President Vladimir Putin had offered "his sincerest words of sympathy and support to families and friends of the victims", while Malaysian Prime Minister Najib Razak said he was "shocked" by the tragedy.
SkyNews reporter Katie Stallard, in Moscow, said media reports suggest the plane came down in an area where there has been recent heavy fighting amid continuing tensions between Russia and Ukraine.
Data from Flightradar24 indicates the plane had just passed the city of Kremenchuk, around 300km (186 miles) from the Russian border, when it disappeared.
Aviation expert Major Charles Hayman told Sky News: "It's highly likely this aircraft was flying along a fault line between Russian and Ukrainian defences.
"It's possible the Ukrainians flapped a bit, thought it was hostile and shot it down. It looks like someone failed to recognise this was a civilian plane."
A spokesman for Boeing said it was aware of reports of MH17's disappearance, while a Foreign Office official added: "We're urgently working to establish what has happened."
The White House said US President Barack Obama had spoken to his Russian counterpart Vladimir Putin and asked his advisers to keep him updated. – skynews