By Adrian Croft

BRUSSELS (Reuters) - NATO chief Anders Fogh Rasmussen said on Monday he saw a "high probability" that Russia could intervene militarily in eastern Ukraine and that NATO detected no sign that Moscow was pulling back thousands of troops from close to the Ukrainian border.

Asked in a Reuters interview how he rated the chances of a Russian military intervention, Rasmussen said: "There is a high probability."

"We see the Russians developing the narrative and the pretext for such an operation under the guise of a humanitarian operation and we see a military buildup that could be used to conduct such illegal military operations in Ukraine," he said.

NATO said last week that Russia had amassed around 20,000 combat-ready troops on Ukraine's eastern border.

However, President Vladimir Putin's spokesman Dmitry Peskov said on Monday that Moscow would send humanitarian aid to Ukraine only if all involved parties agreed to the move.

Russia's annexation of Ukraine's Crimea region in March and its support for separatist rebels in eastern Ukraine have caused the biggest crisis in its relations with the West since the Cold War. NATO has suspended practical cooperation with Moscow.

Russia's Defence Ministry said on Friday it had finished military exercises in southern Russia, around 1,000 km (600 miles) from the Ukraine border, which the United States had criticized as a "provocative" step amid the Ukraine crisis.

However, Rasmussen said the Western military alliance saw no signs of a withdrawal of Russian forces from close to the Ukraine border.

KEEPING OPTION OPEN

"There are thousands of Russian troops, well equipped, and we see it as an indication that the Russian leadership keeps the option open to intervene further militarily in Ukraine," he said. "Any Russian intervention under the guise of a humanitarian mission would be unjustified and illegal."

Asked if NATO, a U.S.-dominated, 28-nation alliance, could get involved militarily in Ukraine if Russian forces did invade, Rasmussen said it was a hypothetical question.

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"However, we are not considering military operations. If the Russians were to intervene further in Ukraine, I have no doubt that the international community would respond determinedly, notably through broader, deeper, tougher economic sanctions that would isolate Russia further," he said.

Accusing Moscow of creating the "disastrous humanitarian situation" in eastern Ukraine, Rasmussen said the best way to solve it would be for the Russians to pull back their troops from the border, "stop the flow of weapons and fighters and money into Ukraine and cease the support for armed separatists and engage in a constructive political dialogue".

Rasmussen also said that NATO would take all steps necessary to defend alliance member Turkey if it were threatened by Islamic State fighters who have made gains in neighboring Iraq and Syria.

"We are very much concerned about the activities of the so-called Islamic State, which is a bunch of terrorists, and it is of utmost importance to stop their advance," Rasmussen said.

"If any of our allies, and in this case of course particularly Turkey, were to be threatened from any source of threat, we won't hesitate to take all steps necessary to ensure effective defense of Turkey or any other ally," he said.

NATO sent Patriot anti-missile batteries to Turkey last year to protect it from any spillover from the Syrian civil war but Rasmussen said Ankara had not asked the alliance for any further help following the Islamic State advance.

The alliance had held internal consultations, at Turkey's request, "on the whole situation in the region", he said.

The United States has begun a campaign of air strikes and humanitarian aid drops in northern Iraq but NATO was not considering any military action against Islamic State, Rasmussen said.

Rasmussen's five-year term as NATO chief will finish at the end of September following a NATO summit in Wales. He will be succeeded by former Norwegian Prime Minister Jens Stoltenberg.

(Editing by Martin Santa and David Stamp)