BRUSSELS, April 2 - Further Russian intervention in Ukraine, following its annexation of Crimea, would be an "historic mistake" that would deepen Russia's international isolation, NATO Secretary-General Anders Fogh Rasmussen said on Wednesday.

"If Russia were to intervene further in Ukraine, I wouldn't hesitate to call it an historic mistake,” he told a news conference after a meeting of alliance foreign ministers, Reuters reported.

“That would lead to further international isolation of Russia. It would have far reaching consequences for the relations between Russia and ... the Western world. It would be a miscalculation with huge strategic implications," he said.

Russia has massed all the forces it needs on Ukraine's border if it were to decide to carry out an "incursion" into the country, and it could achieve its objective in three to five days, NATO's supreme allied commander in Europe, U.S. Air Force General Philip Breedlove said Wednesday.

Calling the situation "incredibly concerning", Breedlove, said NATO had spotted signs of movement by a very small part of the Russian force overnight but had no indication that this was part of a withdrawal to barracks.

Russia's seizure and annexation of Ukraine's Crimea region has caused the deepest crisis in East-West relations since the Cold War, leading the United States and Europe to impose sanctions on Moscow. They have said they will strengthen these if Russia moves beyond Crimea into eastern Ukraine.

NATO military chiefs are concerned that the Russian force on the Ukrainian border, which they estimate stands at 40,000 soldiers, could pose a threat to eastern and southern Ukraine.

"This is a very large and very capable and very ready force," Breedlove said in an interview with Reuters and The Wall Street Journal.

The Russian force has aircraft and helicopter support as well as field hospitals and electronic warfare capabilities - "the entire suite that would be required to successfully have an incursion into Ukraine, should the decision be made," Breedlove said.

"We think it is ready to go and we think it could accomplish its objectives in between three and five days if directed to make the actions."

Russia has said it has no intention of invading its neighbor, although since the toppling of Moscow-backed Ukrainian President Viktor Yanukovych in February it has asserted a right to intervene to protect ethnic Russians if necessary.

Breedlove said Russia could have several potential objectives. These included: an incursion into southern Ukraine to establish a land corridor to Crimea; pushing beyond Crimea to Ukraine's Black Sea port of Odessa; or even threatening to connect to Transdniestria, the mainly Russian-speaking, separatist region of Moldova that lies to the west of Ukraine.

Russia also has forces to the north and northeast of Ukraine that could enter the east of the former Soviet republic if Moscow ordered them to do so, Breedlove said. (rt/ez)