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"You will see deployments at sea, in the air, on land to take place immediately, that means within days," NATO Secretary-General Anders Fogh Rasmussen told a news conference after NATO ambassadors agreed the measures.
NATO has made clear it will not intervene militarily in Ukraine, which is not a NATO member, despite Russia's annexation of Ukraine's Crimea region and a buildup of Russian forces on the Ukrainian border.
It is focusing instead on boosting temporarily its presence in eastern Europe in a drive to reassure allies, such as the ex-Soviet Republics in the Baltics, that NATO would protect them if they ever faced Russian aggression.
The United States and other allies have already sent more planes and ships to the region but NATO ambassadors adopted further steps on Wednesday that will maintain a bigger NATO presence in eastern Europe until at least the end of this year.
Rasmussen said NATO fighter aircraft will fly more sorties over the Baltic region, allied ships will be deployed to the Baltic, the eastern Mediterranean and elsewhere, and allied military staff will be sent to improve NATO's preparedness for training and exercises.
Rasmussen said the defence alliance would reinforce its contingency plans to defend members. More steps to reinforce defences would follow, if needed, in coming weeks and months. Beyond that, NATO officials gave few details.
The measures were welcomed by the Baltic nations but are likely to disappoint Poland which wants NATO forces based permanently on its territory.
Russia says deployment of significant NATO forces in eastern Europe, close to Russia, would violate the 1997 Founding Act, a cooperation agreement between Moscow and the alliance.
Latvian Foreign Minister Edgars Rinkevics said: "It corresponds to the current situation in the region and takes into account potential risks. I don't think this decision will escalate the situation. Those who expected big bases, maybe (they) will say it is not enough, but ... the response is adequate to the current situation."
NATO's top military commander, U.S. Air Force General Philip Breedlove, said the measures were "not a threat to Russia but they are designed to send a clear message that NATO will protect every ally."
Breedlove said he had tried unsuccessfully to contact General Valery Gerasimov, chief of the general staff of Russia's armed forces, on Wednesday to "talk him through" the measures and would try again.
He said NATO had seen no significant change in the numbers of Russian soldiers massed near Ukraine's border, which NATO officials estimate at up to 40,000.
NATO diplomatic sources said the alliance had been careful not to take steps that were too aggressive and could jeopardise talks on Ukraine's future in Geneva on Thursday between Russia, Ukraine, the United States and the European Union.
Breedlove said he would deliver details of the units involved and exercises to NATO headquarters by Thursday morning.
He said the reinforcement of ground forces would not be limited to staff officers advising on exercises. "Multiple nations have approached me even as late as today with offers of ground forces," he said.
Asked what the Ukraine crisis meant for the numbers of U.S. troops in Europe, which have fallen sharply since the end of the Cold War, Breedlove, who also commands U.S. forces in Europe, said: "It is important that we now take a pause and determine what is appropriate for U.S. forces in Europe and I am having those conversations with my capital."
Germany said it would send a command ship, the "Elbe", for a mine defence unit in the Baltic Sea starting from the end of May. It will also send up to six Eurofighter jets starting from September to reinforce air policing over the Baltic states.