KIEV, March 2 - Ukraine mobilized for war on Sunday and Washington threatened to isolate Russia economically after President Vladimir Putin declared he had the right to invade his neighbor in Moscow's biggest confrontation with the West since the Cold War.

"This is not a threat: this is actually the declaration of war to my country," Ukrainian Prime Minister Arseniy Yatseniuk said in English, Reuters reported. Yatseniuk heads a pro-Western government that took power in the former Soviet republic when its Moscow-backed president, Viktor Yanukovych, was ousted last week.

Putin secured permission from his parliament on Saturday to use military force to protect Russian citizens in Ukraine and told U.S. President Barack Obama he had the right to defend Russian interests and nationals, spurning Western pleas not to intervene.

Russian forces have already bloodlessly seized Crimea, an isolated Black Sea peninsula where Moscow has a naval base.

On Sunday, they surrounded several small Ukrainian military outposts there and demanded the Ukrainian troops disarm. Some refused, leading to standoffs, although no shots were fired.

As Western countries considered how to respond to the crisis, the United States said it was focused on economic, diplomatic and political measures, but made clear it was not seriously considering military action.

With Russian forces in control of majority ethnic Russian Crimea, the focus is shifting to eastern swaths of Ukraine, where most ethnic Ukrainians speak Russian as a native language.

Those areas saw more demonstrations on Sunday after violent protests on Saturday, and pro-Moscow activists hoisted flags for a second day at government buildings and called for Russia to defend them.

Kiev said Russia had sent hundreds of its citizens across the border to stage the protests.

Russia has staged war games with 150,000 troops along the land border, but they have so far not crossed.

Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert.

"You just don't, in the 21st century, behave in 19th century fashion by invading another country on a completely trumped-up pretext," Kerry told the CBS program "Face the Nation,” Reuters reported.

He said Moscow still had a "right set of choices" to defuse the crisis. Otherwise, G8 countries and other nations were prepared to "to go to the hilt to isolate Russia".

"They are prepared to isolate Russia economically. The rouble is already going down. Russia has major economic challenges," he said. He mentioned visa bans, asset freezes and trade isolation as possible steps.

Obama discussed the Ukraine crisis in calls with allies, including German Chancellor Angela Merkel and British Prime Minister David Cameron. Cameron said they agreed Russia would pay "significant costs" unless it changed course.

Analysts said U.S. economic sanctions would likely have little impact on Russia unless they were paired with strong measures by major European nations, which have deeper trade ties with Moscow and are dependent on Russian gas.

Ukraine, which says it has no intention of threatening Russian speakers, has appealed for help to NATO, and directly to Britain and the United States, as co-signatories with Russia to a 1994 accord guaranteeing Ukraine's security.

After an emergency meeting of NATO ambassadors in Brussels, the alliance called on Russia to bring its forces back to bases and refrain from interfering in Ukraine.
Despite expressing "grave concern", NATO did not agree on 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. (rt/ez)