British nationals are believed to be among almost 300 passengers on an Malaysian airliner apparently shot down over worn-torn Ukraine, Foreign Secretary Philip Hammond has said.
It is thought at least six British are among the victims.
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The majority of the victims were from the Netherlands, with 154 of the passengers from the country. There were also 80 children on board.
"I'm deeply shocked by this appalling incident and I send my heartfelt condolences to all those who may have lost family and friends. We're determined to get to the bottom of understanding what has happened here," he said.
"As yet we do not have any definitive information about how this incident occurred and I don't want to speculate at this stage. We believe that there must be a UN-led international investigation of the facts."
He said that Britain was prepared to make Air Accident Investigation Branch assets and specialists available to assist xsuch an investigation.
Others on board included 27 Australians, 23 Malaysians, 11 Indonesians, four Germans, four Belgians, three Phillippinos and one Canadian aboard the Boeing 777 jet.
There were a further 47 passengers whose nationality was not yet known. All the 15 flight crew were Malaysian.
Anton Gerashenko, an adviser to Ukraine's interior minister, said on his Facebook page that the plane was flying at an altitude of 33,000 feet when it was hit by a missile fired from a Buk launcher.
A similar launcher was seen by journalists near the eastern Ukrainian town of Snizhne earlier today. The system can fire missiles up to an altitude of 72,000 feet.
Malaysian Airlines said it lost contact with one of its flights over Ukrainian air space. A statement said it had "lost contact of MH17 from Amsterdam. The last known position was over Ukrainian airspace. More details to follow".
The region has seen severe fighting between Ukrainian forces and pro-Russiam separatist rebels in recent days.
A Ukrainian fighter jet was shot down last night by a Russian plane, Ukrainian authorities said, adding to what Kiev says is mounting evidence that Moscow is directly supporting separatist insurgents in eastern Ukraine.
Security Council spokesman Andrei Lysenko said the pilot of the Sukhoi-25 jet hit by the air-to-air missile was forced to bail out.
Pro-Russia rebels alsoclaimed responsibility for strikes on two Ukrainian Sukhoi-25 jets, but Moscow denies Western charges that is supporting the separatists or sowing unrest in Ukraine.
Ukrainian president Petro Poroshenko appeared to blame separatists for the missile strike, saying the "armed forces of Ukraine did not take action against any airborne targets".
Malaysian prime minister Najib Razak said he was launching an immediate probe into the crash.
Malaysia's defence minister Hishamuddin Hussein said there had been no confirmation that the plane was shot down and added he had instructed the country's military to check and get confirmation.
Mr Poroshenko added: "We are sure that those who are guilty in this tragedy will be held responsible."
Meanwhile, a Scot described how he and his wife had been due to be on the ill-fated flight.
Barry Sim and his wife Izzy were forced to switch to a later, KLM flight, because there were no spare seats on the Malaysian Airlines jet at Amsterdam.
Speaking at the airport, Mr Sim said: "You get this sick feeling in the pit of your stomach [...] We started getting butterflies. Your heartbeat starts going.
"There must have been someone watching over us and saying 'you must not get on that flight'.
His wife Izzy said: "We are very loyal to Malaysia Airlines and we always want to fly with Malaysia Airlines.
"But do you know what, at this moment we are so glad to be on that KLM flight rather than that Malaysia Airlines flight," she said.
Mr Sim said: "In my mind lightning never strikes twice in the same place so I am still philosophical that you get on the flight and you go about your life. I know my wife doesn't feel like that. Probably the last thing she wants to do now is fly, especially to Kuala Lumpur."