A Malaysia Airlines plane with 295 people on board is feared to have crashed in the Ukraine amid claims it was shot down.
TV pictures from the scene showed a pall of smoke billowing into the sky apparently from the stricken aircraft.
The plane is thought to be flight MH17 - a Boeing 777-200 travelling from Amsterdam to Kuala Lumpur.
In a tweet Malaysia Airlines said: “Malaysia Airlines has lost contact of MH17 from Amsterdam. The last known position was over Ukrainian airspace. More details to follow.”
Ukraine’s Interior Minister was quoted as saying the plane had been shot down.
The jet would have been flying at high altitude on an intercontinental flight that took it over the crisis hit region of Ukraine, where the authorities have accused Russia-backed separatists of previous attacks on aircraft.
Earlier today 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 alleged episodes mark 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 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.
As the drama unfolded, several other passenger jets were flying through Ukrainian airspace on one of the main routes from Europe to Asia for air traffic.
Given recent events, questions are now likely to be asked about why airlines were still using the route.
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 over eastern Ukraine when it was hit by a missile fired from a Buk launcher.
A similar launcher was reported by journalists near the eastern Ukrainian town of Snizhne earlier today.
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.”