The first bodies recovered from the Malaysia Airlines plane that was downed over Ukraine were flown back on Wednesday to the Netherlands where a national day of mourning has been declared.

Members of the Dutch royal family were due to meet the two military transport planes carrying 40 bodies which will undergo identification before being handed over to families.

The Netherlands lost 193 citizens on the doomed jet that had 298 passengers and crew on board when it was downed last Thursday in eastern Ukraine, in a region controlled by pro-Russian rebels.

Officials at Kharkiv airport held a minute's silence before the coffins were loaded onto the first Hercules plane, which took off at around noon local time. It was due to land in Holland at 1400 GMT.

US intelligence officials said they believe rebels mistakenly shot down the plane that was flying from Amsterdam to Kuala Lumpur with a surface-to-air missile.

"The most plausible explanation... was that it was a mistake", and that the missile was fired by "an ill-trained crew" using a system that requires some skill and training, said a senior intelligence official who spoke on condition of anonymity.

"We've all seen mistakes in the past," the official told reporters, in reference to a Korean airliner downed by a Soviet fighter jet in 1983, and an Iranian passenger plane shot down by US naval forces in 1988.

Meanwhile experts and world leaders have expressed concern that not all the remains have been recovered from the sprawling crash site in rebel-held territory.

"It's quite possible that many bodies are still out there, in the open in the European summer, subject to interference, and subject to the ravages of heat and animals," Australian Prime Minister Tony Abott, whose country lost 28 nationals, said.

The rebels controlling the crash site released the bodies and handed over two black boxes to Malaysian officials only after intense international pressure.

The black boxes were delivered to Britain for expert analysis on Wednesday.

Information war

Russia, which US officials accuse of backing the separatists by providing them with military hardware and training, has faced a hail of international condemnation over the accident.

The crash has spurred an intense propaganda war, with both Ukraine and Russia trading blame, ratcheting up tensions after months of crisis sparked when Kiev turned its back on its former Soviet master in favour of stronger European ties.

Russia denies supporting the rebels who have declared independence in parts of Russian-speaking eastern Ukraine and also says it did not supply the missile system allegedly used to bring down MH17.

But US intelligence officials said Russian claims the Ukrainian government had shot down the plane were "not plausible" noting that the territory was clearly under rebel control.

A senior security official in Kiev claimed that Russia had massed over 40,000 soldiers along its border over the past week.

'Human remains not picked up'

A truce has been declared by rival sides around the crash impact site, but international investigators still face massive obstacles. Dutch officials confirmed receipt of only 200 of the 298 victims' bodies.

International monitors said more remains were left in the vast crash site, littered with poignant fragments from hundreds of destroyed lives.

"There were human remains that had not been picked up," said Michael Bociurkiw, a spokesman for European security body OSCE's mission to Ukraine after visiting the scene.

Kiev said the Netherlands and other countries that lost citizens are proposing to send police officers to secure the site, amid concerns that vital evidence has been tampered with.

Just beyond the crash site, fighting raged on as government troops pushed on with an offensive to wrest control of east Ukraine's industrial heartland from the pro-Moscow separatists.

Local authorities in the besieged city of Lugansk reported Wednesday three civilians killed and 10 injured in the last 24 hours.

Russian President Vladimir Putin on Tuesday pledged to "do everything" to influence the separatists and ensure a full probe into the crash.

At the same time, he put the ball back in Kiev's court, saying that the Ukrainian military offensive in the east was posing a danger to international investigators there.

"We are asked to exert influence on the militants of the south-east (of Ukraine). Of course we will do everything in our power.

"However this would be absolutely inadequate" given fresh attacks by Ukrainian troops, he said.

Putin is staring down fresh European sanctions just a week after the latest set was unveiled over its role in the Ukraine crisis, which has chilled East-West tensions to the lowest point in years.

The emerging giant is widely expected to sink into recession this year, hit by massive capital outflows over the Ukraine crisis.