A Malaysia Airlines passenger aircraft carrying 295 people on board has crashed after apparently being shot down in eastern Ukraine near the Russian border.
Flight MH17 from Amsterdam to Kuala Lumpur was in transit over the war-torn region when it disappeared from radar screens.
Malaysian Airlines said nine British citizens were on board the plane, ITV News reports. The Foreign Office was unable to confirm this.
Graphic photographs have emerged which appear to show bodies and debris strewn across the crash site.
One image appears to show a piece of the wreckage with somebody climbing over the white, blue and red striped fuselage.
The commercial Boeing airliner, a 777-200, departed from Amsterdam at 12.14am local time, 15 minutes alter than scheduled, according to flight records.
It was expected to arrive in Malaysia at 6:10am local time, but did not enter Russian airspace when it was expected to, a Russian aviation source told Reuters.
An adviser to Ukraine's Interior Minister said the plane was shot down.
Anton Gerashenko said on his Facebook page that the plane was flying at an altitude of 33,000 feet over eastern Ukraine when it was hit by a missile fired from a Buk launcher.
And 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".
He added: "We are sure that those who are guilty in this tragedy will be held responsible."
But a spokesman for the rebels said the plane must have been shot down by Ukrainian government troops.
The incident has sparked a fresh international crisis which will put more pressure on Russia to rein in the rebels.
The country has been torn apart by internal strife since the overthrow of the Moscow-backed regime of Viktor Yanukovych, with Russian backed separatists already accused by the authorities in Kiev of shooting down military jets with missiles supplied by Russia.
Russian president Vladimir Putin and his US counterpart Barack Obama have tonight spoken by phone.
Meanwhile, Malaysian prime minister Najib Razak said he was launching an immediate investigation into the crash.
Malaysia Airlines posted on its Twitter feed that it "has lost contact of MH17 from Amsterdam. The last known position was over Ukrainian airspace. More details to follow".
A statement later read: "Malaysia Airlines confirms it received notification from Ukrainian ATC that it had lost contact with flight MH17 at 1415 (GMT) at 30km from Tamak waypoint, approximately 50km from the Russia-Ukraine border.
"Flight MH17 operated on a Boeing 777 departed Amsterdam at 12.15pm (Amsterdam local time) and was estimated to arrive at Kuala Lumpur International Airport at 6.10 am (Malaysia local time) the next day.
"The flight was carrying 280 passengers and 15 crew onboard. More details to follow."
A similar launcher to the Buk, a Soviet era surface-to-air missile system capable of taking down a high altitude aircraft, was reported by journalists near the eastern Ukrainian town of Snizhne earlier today.
The Interfax report said the plane came down 50 km (20 miles) short of entering Russian airspace. It "began to drop, afterwards it was found burning on the ground on Ukrainian territory," the unnamed source said.
A separate unnamed source in the Ukrainian security apparatus, quoted by Interfax, said the plane disappeared from radar at a height of 10,000 metres after which it came down near the town of Shakhtyorsk.
The region has seen severe fighting between Ukrainian forces and pro-Russian 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 also claimed responsibility for strikes on two Ukrainian Sukhoi-25 jets, but Moscow denies Western charges that is supporting the separatists or sowing unrest in Ukraine.
Malaysia's defence minister Hishamuddin Hussein said there had been no confirmation that MH17 was shot down and added he had instructed the country's military to check and get confirmation.
The area is one of the main routes from Europe to Asia for air traffic.
This evening, the Department for Transport in London said flights were now being diverted.
A DfT spokesman said: "Flights already airborne are being routed around the area by air traffic control in the region. Pilots around the world have been advised to plan routes that avoid the area by Eurocontrol, the European organisation for the safety of air navigation."
Airliner tracking sites seemed to indicated that traffic was now steering clear of Ukrainian airspace.
The incident brings tragedy to Malaysia Airlines for the second time this year.
In March, one of its jets disappeared with 227 passengers and 12 crew on board in one of the greatest aviation mysteries of all time.
It would not be the first time a civilian airliner has been mistakenly shot down.
In 1988, an Iran Air flight from Tehran to Dubai was shot down by the US warship USS Vincennes in the Persian Gulf. All 290 on board, including 66 children and 16 crew, died.
In 1983, Korean Air Lines Flight 007 from New York to Seoul via Anchorage was shot down by a Soviet military jet near Sakhalin Island in the East Sea. All 269 passengers and crew were killed. The Soviets initially denied knowledge of the incident but later admitted responsibility, claiming that the aircraft was on a spy mission.
A spokesman for the Foreign Office said: "We are aware of the reports and are urgently working to establish what has happened."
Boeing, who manufactured the aircraft, said: "We are aware of reports on MH17. We're gathering more information.
"Our thoughts and prayers are with those on board MH17, as well as their families and loved ones. We stand ready to provide assistance."
UK Prime Minister David Cameron tweeted: "I'm shocked and saddened by the Malaysian air disaster. Officials from across Whitehall are meeting to establish the facts."
Almost incredibly, Malaysia Airlines finds itself at the centre of a world aviation disaster for the second time this year.
It is only a few months since the Far East carrier was embroiled in what has become one of the great plane mysteries - the disappearance of flight MH370.
Now another Malaysia Airlines Boeing 777 lies wrecked in the Ukraine - seemingly the victim of a missile attack.
It was on March 8 that flight 370, carrying 239 passengers and crew veered far off course for unknown reasons during a flight from Kuala Lumpur to Beijing.
Initially, it was thought that the plane would soon be located. But weeks, and finally months, have passed - with the search areas changed and different theories being expanded - and still there has been no sign of the aircraft.
The search continues as does the heartache for the families of those lost on the flight.
Now today, comes news that of all the carriers to be involved in what appears to be an act of sabotage it should be beleaguered Malaysian Airlines.
Once more their managers have had to tell the world that they have lost contact with one of their aircraft. Now there are some who doubt whether the airline can recover from this.
The BUK missile system is a set of medium range surface-to-air missile systems which were first developed in the Soviet Union and continue to be produced by Russia. A Buk division of Ukraine's armed forces was reportedly relocated to Donetsk region on Wednesday.
Designed to take out cruise missiles, aircrafts, helicopters and short range ballistic missiles, they can reach altitudes of up to 25km (15.5 miles or 82,000ft), according to the manufacturer's website.
Developed by Moscow firm Almaz-Antey, they are thought to have been used during the Russian war with Georgia in the territory of South Ossetia in 2008.
The manufacturer's website, which also lists military equipment including radar and naval missile systems, displays two models of Buk launchers - the Buk-M1-2 and the Buk-M2E.
A description of the Buk-M1-2, which has an altitude target range of up to 25km (15.5 miles or 82,000ft), reads: "The "Buk-M1-2" ADMC is designed to provide air defence for troops and facilities against attacks from current and future high-speed manoeuvring tactical and strategic aircraft, attack helicopters including hovering helicopters, and tactical ballistic, cruise, and air-to-air missiles, in conditions of heavy radio jamming and counter fire; as well as to destroy water and ground surface targets."
Meanwhile, the Buk-M2E "is designed to destroy tactical and strategic aircraft, helicopters, cruise missiles, and other aerodynamic aircraft at any point in their range of operation, along with tactical ballistic and aircraft missiles, and smart air bombs in conditions of heavy enemy counter fire and radio jamming; as well as to attack water and ground surface contrast targets."