Tallinn (AFP) - US President Barack Obama arrived in Estonia early Wednesday to meet Baltic leaders and reaffirm Washington's commitment to the security of ex-Soviet NATO members, as Russia declared NATO a major "threat".

Moscow's surprise declaration came after the Western military alliance announced plans to reinforce defences in eastern Europe because of the Kremlin's perceived stoking of war in Ukraine.

It also comes ahead of a NATO summit in Wales on Thursday at which beleaguered Ukrainian President Petro Poroshenko will lobby Obama for military backing.

Obama will deliver a message of firm NATO support for its newest members from the former Soviet empire on Wednesday when he visits the tiny Baltic state of Estonia, where he will also meet the leaders of Latvia and Lithuania.

As he boarded Air Force One in Washington, the White House said Obama will put Russia on notice that it must not attempt the kind of intimidation and infiltration of NATO's former Soviet members that it has carried out in Ukraine, a non-NATO member.

"NATO has played a leading role and produced ample evidence to indicate that Russia has intervened in ways that grossly violate the territorial integrity of the independent nation of Ukraine," said White House spokesman Josh Earnest.

"It's not OK for large countries to flagrantly violate the territorial integrity of their smaller neighbours."

The Russian national security council's deputy secretary Mikhail Popov said NATO's plan for new fast-response units in eastern Europe was "evidence of the desire of US and NATO leaders to continue their policy of aggravating tensions with Russia".

Popov added that Russia's 2010 military doctrine -- a document that already permits the use of nuclear weapons in case of grave national danger -- would focus more in future on overcoming NATO and its new European anti-missile defence system.

Ukraine on Tuesday reported losing 15 more soldiers in the latest day of clashes with Russian-backed insurgents whose ongoing offensive threatens to stamp Moscow's permanent hold on the eastern half of the ex-Soviet state.

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The United Nations' refugee agency said on Tuesday that the fighting has driven more than half a million people from their homes in addition to claiming an estimated 2,600 lives.

- NATO rapid response unit -

The Ukrainian president's appeal for European military assistance in the face of Russia's alleged dispatch of crack troops into the conflict zone was dismissed at a meeting of EU leaders in Brussels at the weekend.

But NATO chief Anders Fogh Rasmussen said that the 28-nation alliance would endorse the establishment of a force of "several thousand troops" that could be deployed within "very few days" to meet any perceived Russian military movements in eastern Europe.

The New York Times reported the rapid-response unit would be supported by new NATO members such as Poland that were once Soviet satellites but now view Russian President Vladimir Putin with fear and mistrust.

Yet the plan would be of no immediate help to Ukraine's government because the country is not a member of NATO -- a point stressed by Obama in his rejection of calls to involve the US military.

Poroshenko convened his national security and defence council late on Monday to discuss mounting setbacks in the mostly Russian-speaking regions where the army had until recently put rebels on the back foot.

"The situation is difficult but the Ukrainian fighting spirit is stronger than that of the occupants," Poroshenko said in reference to more than 1,000 Russian soldiers that NATO believes the Kremlin has sent across the Ukrainian border in recent days.

Moscow on Monday again denied either sending or planning to deploy troops into eastern Ukraine. Officials dismiss allegations that Russian intervention is being used to carve a land corridor from Russia to the Crimean peninsula -- another Ukrainian region which Russia took over in March.

Separatist commanders have termed Russian soldiers in their ranks as having come while off-duty or on vacation.

That admission prompted Ukrainian Prime Minister Arseniy Yatsenyuk to declare that Kiev would abandon its non-aligned status and seek NATO membership in the coming years -- a development Russia would intensely oppose.

- 'No military solution' -

UN Secretary General Ban Ki-moon responded to the Cold War-style rhetoric by cautioning "there is no military solution" and calling for "political dialogue."

An inconclusive round of European-mediated talks between Kiev and Moscow envoys and a few separatist leaders concluded in the Belarussian capital Minsk with only a tentative agreement to meet again on Friday.

Putin on Sunday hardened Russia's previous position of demanding only talks on autonomy for the region by noting that Kiev would have to discuss "statehood" for the two rebel eastern districts.

Kremlin foreign policy adviser Yury Ushakov also confirmed that Putin had recently told European Commission chief Jose Manuel Barroso in private of Russia's ability to capture Kiev in two weeks if it wished.

The economic cost of the conflict was emphasised on Tuesday when the International Monetary Fund vowed to stand by Ukraine despite the country's economy being crippled so badly that a current $17-billion (13-billion-euro) rescue package will prove too small.