BERLIN (Reuters) - If tries to infiltrate troops into a NATO country, even out of official military uniform as it did before it annexed Crimea, NATO will respond militarily, the alliance's top commander said in an interview published on Sunday.

Soldiers wearing uniforms without national markings were deployed when entered Crimea beginning in late February. Although President initially denied involvement, he admitted in April Russian forces had been active there.

Kiev and Western governments are now waiting to see if will intervene to support besieged rebels in Russian-speaking east. Some defense experts have said Putin might also look to the former Soviet republics in the that have large ethnic Russian minorities, such as or .

U.S. Air Force Gen. , NATO's Supreme Allied Commander , said although NATO had no plans to intervene in non-NATO member , NATO countries in need to start preparing for a possible threat from "little green men" -- referring to soldiers in unmarked uniforms.

"The most important work to prepare a nation for the problem of 'little green men,' or organizing of Russian (speaking) population, it happens first. It happens now," Breedlove said in an interview published online by German newspaper .

"How do we now train, organize, equip the police forces and the military forces of (allied) nations to be able to deal with this?" he said, according to a transcript of his remarks in English provided by NATO.

"If we see these actions taking place in a NATO nation and we are able to attribute them to an aggressor nation, that is Article 5. Now, it is a military response," he said.

NATO's mutual defense clause says an attack on one member state is considered an attack against the alliance as a whole.

The crisis has dragged relations between and the West to their lowest point since the Cold War and set off a round of trade restrictions that are hurting struggling economies in both and .

NATO has taken a series of steps to reinforce its military presence in since the crisis began. Longer-term measures are expected to be agreed at a NATO summit in in September.