* NATO ambassadors hold emergency session to discuss Crimea
* Rasmussen warns Russia it threatens Europe's security
* Diplomats see limited scope for NATO response
* NATO calls on Russia to send troops back to bases (Recasts with end of NATO meeting)
By Luke Baker and Justyna Pawlak
BRUSSELS, March 2 (Reuters) - Russia's seizure of Crimea is a threat to peace in Europe and the situation must be "de-escalated", NATO's secretary-general said on Sunday, but the alliance failed to agree any major steps to rein in Russia during emergency talks in Brussels.
Speaking before chairing a meeting of ambassadors from the 28 NATO member states, known as the North Atlantic Council, Anders Fogh Rasmussen warned that Russia's actions were unacceptable and could destabilise the continent.
"What Russia is doing now in Ukraine violates the principles of the United Nations charter," Rasmussen told reporters.
"It threatens peace and security in Europe. Russia must stop its military activities and its threats."
After the talks, NATO called on Russia to send troops back to bases and refrain from interference elsewhere in Ukraine.
But despite expressing "grave concern", NATO didn't agree any significant measures to apply pressure to Russia, with the West struggling to come up with a forthright response that does not risk pushing the region closer to military conflict.
Rasmussen said only that Russia and Ukraine should submit to international mediation and said NATO members could hold further meetings on the issue, including with Russia. There was no mention of scaling back any cooperation with Moscow.
"NATO allies will continue to support Ukrainian sovereignty, independence, territorial integrity, and the right of the Ukrainian people to determine their own future, without outside interference," Rasmussen said.
The stand-off has created the greatest moment of tension between Russia and the West since the collapse of the Soviet Union in 1991, an event Russian President Vladimir Putin once called the worst geopolitical catastrophe of the 20th century.
NOT RILING SENTIMENT
"I think we must be careful not to give the Russians anything that could rile up pro-Russian sentiment in Crimea," one senior NATO diplomat said before the meeting.
Despite a 90-minute phone call between President Barack Obama and Putin on Saturday, and other calls to the Kremlin by European leaders, Russia shows no sign of backing away from its de facto occupation of Crimea and influence in east Ukraine.
U.S. Secretary of State John Kerry told Russia it could face targeted sanctions including visa bans, asset freezes and trade isolation if it did not back down, and said major world powers were ready to isolate Moscow.
EU foreign ministers will hold an emergency meeting on the crisis on Monday, but are not expected to agree any sanctions. Instead, several diplomats indicated there would be a push for high-level mediation.
Ukraine's ambassador to NATO, Ihor Dolhov, expressed some hope that the alliance was taking steps towards resolving tensions with Russia, but gave no details.
"I believe this is the beginning of developing additional measures which could be taken in the framework of the NATO-Ukraine Commission which could help us to de-escalate the situation," he told reporters after meeting his NATO peers.
Moscow has said it is merely protecting the lives of Russian-speaking nationals, and appears to be calculating that the West cannot afford to risk a wider conflagration by taking anything approaching military action.
While Ukraine is associated with NATO, it is not a member and therefore cannot invoke the alliance's most powerful diplomatic tool, known as Article 5, which states that an attack against one member is an attack against all.
Given those limitations, the strongest response that can be expected in the coming days is for the United States to move some warships into the Black Sea, military experts said.
"NATO can only say what 28 nations allow it to say," said Karl-Heinz Kamp, academic director of the Federal Academy of Security Policy in Berlin. "There are member states which are more pro-Russia than others."
Several NATO and European Union member states depend almost entirely on Russia for energy, giving them reason to maintain solid relations even if they deplore its actions.
"Wars aren't very popular at this moment," said Tim Ripley, a military expert with Jane's Defence Weekly magazine.
Yet Western credibility may be on the line.
"If the Russians take over Crimea, it would humiliate the West and show it to be a paper tiger, unwilling to protect a European country against outside aggression," said Ripley.
Short of a military response, NATO could cut cooperation with Russia, with which it has frequent contact at ministerial level and has conducted joint military exercises.
Political or economic sanctions are also possible, although the European Union has to decide such measures unanimously, which could prove difficult. Cyprus alone could conceivably veto pan-EU sanctions against Moscow.
Alternatively, the West could try diplomatic isolation of Russia, which is due in June to host a summit in Sochi of the Group of Eight leading economic powers. Already, four G8 members have halted preparations for the gathering and diplomats have talked about suspending Russia. (Additional reporting by Robert-Jan Bartunek and Adrian Croft; Writing by Luke Baker; Editing by Alistair Lyon)
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