Ukraine's Western-backed leaders on Thursday blamed Russian agents and the country's ousted president for organising two days of bloodshed during February protests that claimed nearly 90 lives.

The explosive allegations were levelled only moments before Russia responded to the new course taken by the ex-Soviet neighbour by hiking the price it must pay for gas shipments to what Ukrainian officials say is the highest rate for any European state.

Washington reacted by warning Russia that "a country should not use supply and pricing terms as tools of coercion to interfere in Ukraine or elsewhere," said White House spokesman Jay Carney.

Moscow also lashed out at its old Cold War nemesis NATO for building up the defences of ex-Communist and Soviet nations that have felt threatened by Russia's recent annexation of Crimea and massive buildup of forces near Ukraine.

The fierce East-West fight for Ukraine's future has exposed the deep divide that splits the nation of 46 million between those who see themselves either as culturally tied to Russia or as part of a broader Europe.

Those tensions exploded on February 18 when gunshots in the heart of snow-swept Kiev heralded the onset of pitched battles between riot police and protesters -- some armed with nothing more than metal shields -- that left scores dead.

Both sides have blamed the other for starting the violence, but there had been no formal probe results unveiled until acting interior minister Arsen Avakov presented his initial findings to reporters on Thursday.

Avakov's conclusion was decisive and potentially devastating for the new leaders' relations with Russian President Vladimir Putin.

He said that deposed president Viktor Yanukovych had issued the "criminal order" to fire at the protesters while agents from Russia's Federal Security Service (FSB) helped him plan and carry out the assault.

"FSB agents took part in both the planning and execution of the so-called anti-terrorist operation," Ukrainian Security Service head Valentyn Nalyvaichenko told the same press briefing.

An FSB spokesman told Russia's state-run RIA Novosti news agency that Ukraine's allegations were patently false. Russian Foreign Minister Sergei Lavrov for his part said "huge amounts of evidence" contradicted Kiev's claims.

Yanukovych fled to Russia only days after the carnage and is now wanted in Kiev for allegedly ordering police to open fire against the crowds -- a charge he denies but that is likely to keep him out of Ukraine for years to come.

"Former president Yanukovych will be prosecuted," Ukrainian Prime Minister Arseniy Yatsenyuk told the BBC. "He is accused of mass murder and we will bring him to justice."

- Russia ups Ukraine payments -

The raging security crisis on the eastern edge of the European Union has been accompanied by months of economic pressure that Russia had poured on Ukraine in a seeming effort to force its leaders to reverse their Westward course.

Russia's state energy giant Gazprom -- long accused of being wielded by the Kremlin as a weapon against uncooperative neighbours -- on Tuesday hiked the price it charges Ukraine for natural gas shipments on which its industries depend by 44 percent.

The punitive but largely expected step eliminated a price discount that Putin had extended to the old government in December in reward for its decision to reject closer EU ties.

But Ukraine saw the price it must pay for 1,000 cubic metres of gas jump by another $100 to $485.50 following a failed round of negotiations in Moscow with the chief executive of Russia's state energy firm Gazprom.

Moscow argues that a $100 rebate it awarded Kiev in 2010 in return for its decision to extend a lease under which the Kremlin keeps its Black Sea Fleet in Crimea no longer applied because the peninsula was now a part of Russia.

Kiev has vowed to contest the new charge -- a warning that threatens a repeat of the 2006 and 2009 halts in gas supplies to Ukraine that also affected many of Russia's other European clients.

"This is an unacceptable price for Ukraine because it is a political price," said Energy Minister Yuriy Prodan.

- NATO defends security boost -

Europe's worst security crisis in decades appeared to be only gaining momentum after NATO boosted the air power of Eastern European nations that Putin still views as part of Russia's strategic domain.

NATO chief Anders Fogh Rasmussen Thursday defended that move against Russian claims it violated international law.

"I'm actually surprised that Russia can claim that NATO has violated its commitments because Russia is violating every principle and international commitment it has made," Rasmussen said.

"First and foremost the commitment not to invade other countries," he added.

The 28-nation alliance has said firmly it did not intend to get militarily involved in Ukraine no matter what Russia did.

But NATO has vowed to review both its immediate strategy and historic mission after conceding that a Russian strike against Ukraine -- a non-NATO member with an ill-equiped and underfunded army -- could be both decisive and quick.

The US Air Force this week sent 10 F-15 fighters to help NATO expand its military presence in the three tiny ex-Soviet Baltic nations.

bur-zak/boc/lc

Gazprom