By Richard Balmforth

KIEV (Reuters) - Ukraine said on Friday it would seek the protection of NATO membership after what Kiev and its Western allies say is the open participation of the Russian military in the war in Ukraine's eastern provinces.

Formal Ukrainian membership in the Western military alliance - which would come with the full protection of a mutual defence pact with the U.S. superpower - remains an unlikely prospect, at least in the near future.

But by announcing that it is seeking it, Kiev is taking its most decisive step yet to pursue Western military protection from what it now describes as an invasion by its neighbour.

NATO's secretary general said he respected Ukraine's right to seek membership, and accused Russia of blatantly and illegally intervening in eastern Ukraine.

Moscow denies its forces are fighting to support pro-Russian rebels who have declared independence in eastern Ukraine, but the rebels have all but confirmed it, saying thousands of Russian troops have fought on their behalf while "on leave".

The arrival of what Western governments say are armoured columns of Russian troops on a new front in recent days has tipped the balance towards the rebels after weeks in which Ukrainian forces appeared to be gaining the upper hand.

Prime Minister Arseny Yatseniuk told a government meeting on Friday the cabinet would "bring before parliament a law to scrap the non-aligned status of the Ukrainian state and establish a course towards membership of NATO."

Were NATO to contemplate extending its mutual defence pact to Ukraine, it would be the biggest change in the security architecture of Europe since the 1990s.

After the Cold War, NATO defied Russian objections and expanded to grant its security guarantees to former Communist countries like Poland, Hungary and Romania. But it largely stopped at the border of the former Soviet Union, admitting only the three tiny Baltic states Latvia, Lithuania and Estonia.

NATO denied Ukraine a fast track towards membership in 2008 when a previous pro-European government in Kiev tried to pursue closer ties with the alliance.

Ukraine's pro-Russian president Viktor Yanukovich, who took power in 2010 and was toppled in a revolution earlier this year, had pursued non-aligned status. President Petro Poroshenko, who was elected to succeed him in June, had also said he did not back joining NATO because there was no popular support for it.

But with the conflict in the east escalating since then, Ukrainian officials say public support for joining the alliance is mounting, and they have increasingly spoken about needing the formal protection of the alliance.

Despite NATO's concern over the situation in Ukraine, the 28-nation alliance has said repeatedly it has no intention of intervening militarily to protect the country, which it is not obliged to defend as a non-member.

Speaking in Brussels on Friday, NATO Secretary-General Anders Fogh Rasmussen said Russia's direct military operations in Ukraine were in blatant violation of Ukraine's sovereignty and territorial integrity.

"Despite Moscow’s hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine," Rasmussen said.

"This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation," he added.

He said NATO would fully respect any decision by the Ukrainian parliament to abandon Ukraine's non-aligned status and set out to seek NATO membership.

Yatseniuk said earlier this week that Ukraine hoped for "practical help" from the alliance when it holds a summit in Wales next week which will be dominated by the Ukrainian crisis.

(Writing By Richard Balmforth)