Ukraine's president openly challenged the Kremlin on Thursday by submitting a bill allowing the former Soviet republic to join NATO and make the Western alliance its defender from Russian threats.

The high-stakes decision is certain to provoke a Kremlin outcry and may further complicate the fate of delicate peace talks between Kiev and Russian-backed gunmen tentatively scheduled for Sunday.

President Petro Poroshenko's measure would revoke Ukraine's non-aligned status -- a classification given to neutral states such as Switzerland that refuse to join any military alliance and thus play no active part in wars.

Ukraine became such a country in 2010 under strong pressure from Russia. It had sought NATO membership for parts of the early post-Soviet era but -- its once-mighty army in ruins and riven by corruption -- was never viewed as a serious candidate.

Yet Poroshenko -- his seven months in power defined by a separatist war and a Cold War-style standoff between Moscow and the West -- has made the bloc's military protection one his top foreign policy priorities.

He and NATO both accuse Russian President Vladimir Putin of not only funding and arming but also backing up with elite troops the militias who have been fighting Kiev's forces in the industrial east since April.

Moscow denies the charge and claims NATO satellite imagery purporting to show the movement of tanks and heavy weapons across its border into the war zone is a fabrication.

Putin said at his annual press conference Thursday that Russian soldiers in eastern Ukraine were volunteers "answering the call of the heart" who were never sent their directly by the Kremlin.

He never answered a question about how many armed Russians may be in eastern Ukraine today. Both Kiev and NATO put their number at a few a thousand -- down from 10,000 a few months ago.

Poroshenko said in a message to parliament that the bill's adoption was "urgent" and must be put on the agenda of an emergency session.

"Ukraine's non-aligned status... proved to be an ineffective means of ensuring the country's security from foreign aggression and pressure," an explanatory note to the bill quoted on the president's website said.

"Ukraine's long-term occupancy of a 'grey' buffer zone between powerful systems of collective security is an additional threat."

- Polls show NATO backing -

Pollsters at Kiev's respected Razumkov Centre showed support for NATO membership soaring to 45 percent in October from less than 25 percent a year earlier.

Only 37 percent opposed joining the alliance in the latest survey.

"Putin's actions are the main reason for the shift," Razumkov Centre sociologist Andriy Bychenko told AFP.

NATO discussed offering Ukraine membership in the wake of Russia's 2008 war with its southern neighbour Georgia.

But Brussels never outlined a firm timeline for the membership of either Ukraine or Georgia out of fears that this might provoke Russia into even more hostile behaviour.

Former president Viktor Yanukovych picked the new non-alignment policy after striking a lucrative lease agreement with Russia for its Black Sea Fleet in Crimea -- a peninsula seized by Moscow in March.

Putin had sought to pull Ukraine into a new military and political bloc that he is slowly forging with other small former Soviet republics and that he hopes may one day be joined by India and Iran.

That dream effectively shattered when Yanukovych was ousted from power in a popular February uprising that Putin has branded "a coup".

Putin has since made Ukraine's neutrality one of the main conditions of any peace deal Kiev may strike with the eastern guerrillas.