World leaders on Thursday demanded an international investigation into the downing of a Malaysian airliner, which crashed on Thursday with 298 people on board in strife-torn east Ukraine.

Britain has called for an UN-led investigation and is seeking an emergency meeting of the UN Security Council to discuss the crisis in Ukraine's rebel-held east -- which NATO described as "more and more dangerous".

"We believe that there must be a UN-led, international investigation of the facts," Foreign Secretary Philip Hammond told a press conference on Thursday.

US officials believe the plane was shot down by a surface-to-air missile, while comments attributed to a pro-Russia rebel chief suggested his men may have downed Malaysia Airlines flight MH17 by mistake, believing it was a Ukrainian army transport plane.

"The facts of what happened and who was responsible must be quickly established," the presidents of the European Council and Commission, Manuel Barroso and Herman Van Rompuy, said in a joint statement. "The inquiry must be rapid and complete."

UN chief Ban Ki-Moon echoed calls for a "full, transparent and international investigation".

NATO Secretary General Anders Fogh Rasmussen said those guilty of bringing down the airliner must be brought to justice.

"This is another painful illustration of the reason why it is so urgent to end this conflict. The instability in the region caused by the separatists supported by Russia is creating a more and more dangerous situation," he added.

Malaysia dispatched a team of investigators to find out what happened to flight MH17, and Prime Minister Najib Razak said he had spoken to Ukraine's President Petro Poroshenko who had vowed a "full, thorough and independent" probe.

President Barack Obama called the crash "a terrible tragedy" and said he has asked his national security team to "stay in close contact with the Ukrainian government".

"The United States will offer any assistance we can to help determine what happened and why."

US Vice President Joe Biden repeated Washington's offer during a phone call to Ukrainian President Petro Poroshenko, a US official said.

- Ukraine 'bears responsibility' -

Russian President Vladimir Putin said he had also asked Russian military chiefs to "provide all necessary help to shed light on this criminal act".

But he also put responsibility for the disaster squarely on Ukraine.

"There is no doubt that the country on whose territory this terrible tragedy happened bears responsibility," said Putin, quoted by Ria Novosti news agency.

"This tragedy would not have happened if there was peace in the country, if military operations had not resumed in the south-east of Ukraine," he said.

EU foreign policy chief Catherine Ashton called on "all parties in the region to allow full access to the (crash) site."

British Prime Minister David Cameron said government officials were meeting "to establish the facts".

He tweeted: "I'm shocked and saddened by the Malaysian air disaster."

French President Francois Hollande offered "all my solidarity" with relatives of the crash victims, and echoed calls for a wide-ranging investigation.

Germany also demanded that "everything be done to shed light on this disaster as quickly as possible".

Malaysian Airlines flight MH17 disappeared from radar screens in eastern Ukraine at around 1415 GMT, the carrier said, hours after the Boeing 777, bound for Kuala Lumpur, had taken off from Amsterdam's Schiphol airport.

About half of the 298 on board were Dutch nationals, while about 50 others remained unidentified. (AFP)