The downing of the MH17 flight over rebel territory in Ukraine has put Moscow's support of the separatists under more scrutiny than ever amid allegations the plane was blasted out of the sky with a Russian-supplied missile system.

The US, whose relations with Moscow have dropped to a post-Cold War low over the Ukraine crisis, has led the charge.

"It's pretty clear that this is a system that was transferred from Russia in the hands of separatists," Secretary of State John Kerry said on Sunday.

But Moscow has denied the allegations, with a senior member of Russia's military general staff, Lieutenant-General Andrei Kartopolov, dismissing images that allegedly show Buk missile systems being transferred from Russia into Ukraine as fake.

"I want to stress that Russia did not give the rebels Buk missile systems or any other kinds of weapons or military hardware," he told reporters on Monday at a briefing, armed with slides, charts and images.

Observers say that separatists fighting Ukrainian troops in the east of the ex-Soviet republic would not be able to do so without support from Moscow.

"Without Russia, the rebels wouldn't have tanks or heavy artillery. The decisions (to supply rebels with weapons) are made at the governmental level," said Alexander Konovalov, president of the Institute of Strategic Assessments.

"This is being done under pressure from political groups that are even more anti-Western than (Russian President Vladimir) Putin is," he added.

- 'Participant in the conflict' -

The Malaysian plane crash, which killed 298 people flying from Amsterdam to Kuala Lumpur on Thursday, has increased perceptions of Russia's involvement with the rebels, said Maria Lipman, a political analyst with the Carnegie Moscow Centre.

"Russia has become a participant in the conflict in the eyes of the world, one who can control the rebels and with whom lies responsibility for this tragedy," Lipman said.

The Kremlin has always kept a certain level of official distance from the separatists -- Putin has never met with the leaders and did not recognise the independence of the self-proclaimed "Donetsk People's Republic" and "Lugansk People's Republic".

Russia even urged the separatists not to hold a referendum on secession from Ukraine -- a call they ignored, creating what observers say was an impression of disobeying Moscow's orders.

The Kremlin also snubbed the rebel leaders' calls to absorb Ukraine's separatist eastern regions into Russia and to send in peacekeeping troops when the armed conflict with Kiev intensified.

"It was admitted that Russia is involved in the conflict, but the extent of the involvement was concealed," said Lipman.

"Russia did not hide that consultations were held with rebel leaders, some of whom are also Russian citizens, and are partly financed by Russian businessmen."

Russian newspapers and television channels close to the Kremlin have sided with the pro-Russian rebels since the beginning of their uprising -- something they would not have done without Moscow's blessing.

When one of the separatist leaders, Denis Pushilin, claimed in June that he was received by Putin's powerful advisors, Vladislav Surkov and Sergei Glazyev, this was widely reported in the Russian media although not confirmed by the Kremlin.

Russian media has regularly reported on links between businessmen close to the Kremlin and rebel leaders such as the self-appointed prime minister of the Donetsk Republic, Alexander Borodai, and his defence minister Igor Strelkov -- both Russian citizens.

Unconfirmed media reports suggested that the rebel leaders are backed by a so-called "party of war" within the Russian leadership -- allegedly led by the Deputy Prime Minister Dmitry Rogozin and Defence Minister Sergei Shoigu.

But despite the increased scrutiny, Putin has very limited room for manoeuvre between domestic political pressure and criticism from the United States and Europe, said Lipman.

"He can't give in to the West. Public opinion and apparently political elites expect something else from him - an anti-Western stance and support for pro-Russian rebels," she said.