At least six British nationals are believed to be among nearly 300 people on board a Malaysian airliner apparently shot down over war-torn Ukraine.
Foreign Secretary Philip Hammond called for an international investigation to establish what happened to Flight MH17 which crashed into territory held by pro-Russian separatists as it travelled from Amsterdam to Kuala Lumpur.
The chief executive of Amsterdam's Schiphol Airport, Jos Nijuis, said six British nationals were known to be among the 295 people on board the Boeing 777-200 - although the nationalities of more than 90 people have yet to be identified.
Speaking at the Foreign Office following emergency talks with ministers and officials, Mr Hammond said they were still working through the passenger data to establish how many Britons there were.
"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 an investigation.
US vice president Joe Biden said, however, it did appear the aircraft had been shot down - while US officials said their intelligence analysis showed it had been hit by a surface-to-air missile.
"I say 'apparently' because we don't actually have all the details. I want to be sure of what I say. Apparently, have been shot down. Shot down, not an accident. Blown out of the sky," Mr Biden said.
The United Nations Security Council will hold an emergency meeting on Ukraine tomorrow.
Ukraine's UN ambassador Yuriy Sergeyev tweeted after the crash that "Ukraine will present the evidence of Russian military involvement into the Boeing crash. This crime should be fully investigated".
The disappearance of the airliner from the radar screens as it flew over eastern Ukraine prompted immediate allegations and counter-allegations with the government in Kiev and the separatist rebels blaming each other.
The rebels insisted that they did not have the equipment capable of bringing down an aircraft which was flying at such high altitude at more than 30,000 feet.
However Ukraine's security services produced what they said were two intercepted telephone conversations that they said showed rebels were responsible.
In the first call, the security services said, rebel commander Igor Bezler told a Russian military intelligence officer that rebel forces shot down a plane today.
In the second, two rebel fighters - one at the scene of the crash - said the rocket attack was carried out by a unit of insurgents about 15 miles north of the crash site.
Graphic images from the crash scene near the village of Grabovo showed a pall of thick smoke hanging above the site and bodies among the charred wreckage.
Malaysian prime minister Najib Razak said "no stone can be left unturned" in the effort to find out what happened.
" If it transpires that the plane was indeed shot down, we insist that the perpetrators must swiftly be brought to justice," he said.
He said the Ukrianian authorities had agreed to open negotiations with the rebels to create a safe corridor to the crash site.
The authorities at Schiphol said that more than half the people on board - 154 - were known to be Dutch nationals. There were also 23 Malaysians, 11 Indonesians, four Germans, four Belgians, three Filipinos and one Canadian.
Prime Minister David Cameron said he was "shocked and saddened" by the tragedy, while Ukrainian president Petro Poroshenko described it as a "terrorist act".
Ukraine 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.
The Ukrainian government immediately blamed the separatists with President Poroshenko declaring the "armed forces of Ukraine did not take action against any airborne targets".
Anton Gerashenko, an aide to the Ukrainian 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 Soviet era surface-to-air missile system capable of taking down a high altitude aircraft.
A similar launcher was reportedly seen by journalists near the eastern Ukrainian town of Snizhne earlier today.
But a spokesman for the rebels said the plane must have been shot down by Ukrainian government troops while Russian president Vladimir Putin laid the blame for the crash on Ukraine.
"This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in south-east Ukraine. And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy," he said.
The incident will prompt questions as to why the route over Ukraine was still being used given previous incidents.
A UK Civil Aviation Authority (CAA) spokesman said: "The Ukrainian authorities are responsible for managing their airspace and the UK or other countries cannot enforce airspace restrictions in the area.
"However, the CAA has previously issued advice to UK airlines on operating in this area and following this incident, Eurocontrol (the European air traffic control body) has issued advice to airlines to plan routes that avoid the area."
The Department for Transport in London said flights were now 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," a DfT spokesman said.
Last night the Ukrainian authorities said one of their fighter jets was shot down by an air-to-air missile from a Russian plane and Ukrainian troops were fired upon by missiles from a village inside Russia.
The episodes add to what Ukraine says is mounting evidence that Moscow is directly supporting separatist insurgents in eastern Ukraine who have substantial quantities of powerful weapons.
Ukraine said a military transport plane was shot down on Monday by a missile fired from Russian territory. Security Service chief Valentyn Nalyvaichenko said he had "unconditional evidence" that Russia was involved in downing that aircraft.
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 Labour MP called for the House of Commons to delay its summer recess, which is due to begin on Tuesday, if it is found that the missile used to down the aircraft was supplied by the Russian government.
Bassetlaw MP John Mann said: "This is a very troubling development which raises serious issues that Parliament will want to consider. It cannot do this if it is not sitting."