A number of Americans are feared dead after a plane thought to have been flying in or near an unsafe zone came down over Ukraine.
A plane which crashed in eastern Ukraine with 295 people on board was reportedly shot down as it flew near airspace deemed unsafe for passenger jets.
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 contact was lost.
An adviser to the Ukrainian interior ministry told the Interfax news agency the Boeing 777 was brought down by a Buk ground-to-air missile, killing all 280 passengers and 15 crew members.
Foreign Secretary Philip Hammond said a number of Britons are feared dead, although the passenger manifest for flight MH17, thought to also include 23 Americans, has not yet been released.
Plumes of thick, black smoke could be seen rising high into the air near the village of Grabovo, Donetsk.
The plane, which one eyewitness said split in two on impact, is almost unrecognisable in pictures of the crash site, with burning wreckage scattered across a vast area.
Emergencies Ministry members work at the site of a Malaysia Airlines Boeing 777 plane crash in the settlement of Grabovo in the Donetsk region
Emergency services search through the wreckage of the crashed plane
A spokesman for Malaysia Airlines, still reeling from the loss of flight MH370 in March, said the plane disappeared from radar as it flew over Ukrainian airspace.
Officials in Kiev were quick to deny any involvement, with Ukrainian President Petro Poroshenko lamenting what he called an “act of terrorism”.
Sky’s Katie Stallard, in Moscow, said Igor Strelkov, the commander of the pro-Russian Donetsk People’s Republic, appeared to have boasted about the incident on social media.
In one deleted message recovered by Sky News, he allegedly wrote: “We warned you not to fly over our sky.”
However, separatist leader Alexander Borodai said the aircraft was shot down by government forces – a claim backed by another separatist, who told the Reuters news agency rebels do not have weapons capable of shooting down a plane at such height.
Sky’s Mark White, citing aviation sources, said the aircraft appeared to have been flying close to a block of airspace deemed “unsuitable for civilian aircraft”.
Air traffic controllers confirmed the plane was flying in open airspace, about 1,000 feet (300 metres) above a restricted zone.
“It raises questions about why the plane was in an area it had been advised not to fly through,” Stone said.
“Did it stray into that area by accident or did the pilot decide it was a risk worth taking, perhaps as a fuel saving measure?”
Part of the wreckage of a Malaysia Airlines Boeing 777 plane is seen after it crashed near the settlement of Grabovo in the Donetsk region
Debris from the Malaysia Airlines plane was strewn over a vast area
Stallard said the plane came down in an area that had seen heavy fighting in recent days, as tensions between Russia and Ukraine continue.
Data from Flightradar24 indicates the plane, which took off from Schiphol airport at 12.15pm local time, 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 possible the Ukrainians flapped a bit, thought the plane was hostile and shot it down.
A map showing the location of Donetsk in Ukraine
“It looks like someone failed to recognise this was a civilian aircraft.”
The Kremlin said Russian President Vladimir Putin had offered “his sincerest words of sympathy and support”, while Malaysian Prime Minister Najib Razak said he was “shocked” by the tragedy.
Meanwhile, relatives of the victims of the MH370 tragedy also released a statement, saying: “Who would do such poisonous thing to a civil aeroplane?
“Passengers on board are ordinary people, just like our relatives. Why let them experience the torture? Why let other people feel the same pain as we do?”
The disaster is the latest in a line of reported attacks on planes in Ukrainian airspace and comes a day after one of the country’s Sukhoi-25 fighter jets was shot down.
:: Malaysia Airlines has set up an emergency line, 00 6 037 884 1234, for people worried their relatives may be on the flight.