With Russia resolute on defending its right to takeover Ukraine's Crimean region, all eyes are on President Obama as he struggles to build a united response with western allies.

Russian President Vladimir Putin in a rambling one-hour press conference early Tuesday appeared defiant, called the uprising in Ukraine that ousted President Yanukovych “an anti-constitutional takeover” and “an armed seizure of power.”

He denied that Russian troops are on the ground in Crimea despite evidence to the contrary and predicted the situation would not descend into full-blown war with Ukraine, while also refusing to rule out the use of Russian military force in the future.

Russia “reserves the right to use all means at our disposal to protect” ethnic Russians in the south and east if they are threatened, he asserted.

“I want you to understand me clearly,” he said. “If we make such a decision, it will only be for the protection of Ukrainian citizens.”

Putin also took a direct shot at President Obama, accusing the United States of meddling in internal Ukrainian affairs “from across the pond in America as if they were sitting in a laboratory and running experiments on rats, without any understanding of the consequences.”

Obama countered that Russia is the one interfering with Ukraine's sovereignty. He urged Moscow to withdraw troops from Ukraine and said their moves had “not been a sign of strength.”

“I actually think this has not been a sign of strength but rather is a reflection that countries near Russia have deep concerns and suspicions about this kind of meddling, and today they may have pushed many countries further way from Russia,” he said Tuesday at a budget event at a Washington, D.C. school.

The president also dismissed Russia’s suggestion that it had intervened in Ukraine to protect ethnic Russians.

“The fact that we are still seeing soldiers out of their barracks in Crimea is an indication that what's happening there is not based on actual concern for Russian nationals, Russian speakers inside Ukraine, but based on Russia's use of military force to exert influence over a neighboring country,” said Obama. "That is not how international law is supposed to operate.”

But Obama's response has been anything but quick and decisive, and already there are signs of the difficulty of convincing America's European allies to act in lock-step to isolate Russia economically and diplomatically.

After Secretary of State John Kerry went on the Sunday talk shows and suggested that Russia should be kicked out of the G8, Germany's finance minister reportedly told a German news service his government disagreed with the move. Germany relies heavily on Russian natural gas for its energy needs and immediately appeared reluctant to take tough stands against Putin.

"[Obama] does need to step up and provide a level of leadership at a time when that's not going to be easy,” said Charles Kupchan, a senior fellow on the Council on Foreign Relations and professor of international affairs at Georgetown University.

“Getting a united front on how to confront Russia” is going to be difficult, he said, “but I do think it's important for the U.S. and the Europeans to be walking in lockstep.”

The Treasury Department on Tuesday morning announced an economic assistance package that will help the Ukrainian government restore its financial stability and is working with Congress to provide $1 billion in loan guarantees to Kiev. In addition, the International Monetary Fund is developing an assistance package.

Kerry and Assistant Secretary of State Victoria Nuland are in Kiev on Tuesday as a show of support to the fledgling Ukrainian government.

“The United States reaffirms our commitment to Ukraine's sovereignty and territorial integrity, according to international law,” Kerry said at a news conference outside of the American embassy in Kiev. “We condemn the Russian Federation's act of aggression, and we have throughout this moment evidence of a great transformation taking place. And in that transformation, we will stand with the people of Ukraine.”

Vice President Joe Biden worked the phones Monday, calling Russian Prime Minister Dmitri Medvedev to urge Russia to pull troops out of Crimea, and new Ukrainian Prime Minister Arseniy Yatsenyuk and Estonian President Toomas Ilves to consult on the international effort to de-escalate the situation, the White House said.

The growing fears over regional instability were highlighted Monday when NATO scheduled a Tuesday meeting after Poland, fearing its territorial integrity and political independence could be threatened, requested the security group confer.

“The developments in and around Ukraine are seen to constitute a threat to neighboring allied countries and having direct and serious implications for the security and stability of the Euro-Atlantic area,” NATO said in a statement Monday afternoon.

The world is watching whether Putin's takeover of Crimea was meant as a face-saving compensation after he and former Ukrainian President Viktor Yanukovych effectively lost the bulk of Ukraine, or whether he plans to expand Russia's foothold throughout the country.

“From the point of view of the United States and most of the world, the preferred goal is to see the Russians be persuaded to leave Crimea in exchange, say, for some participation in an economic package and some protection for Russian speakers,” said Richard Haass, president of the Council on Foreign Relations, in an interview posted on the organization's website.

Other experts believe Crimea is just too important for Russia to give up and the real goal is to prevent Putin from pushing troops into other regions of Ukraine — a move that would threaten countries throughout the Balkans.

“At this point I don't think it matters if it's an independent Crimea or a Crimea controlled by Russia,” said Kupchan.

Putin is lashing out and trying to save face, Kupchan said, but if he is emboldened and “starts to mess around in Eastern Ukraine — then we really have a huge, full-blown crisis on our hands, but I don't think he's going to do that.”

While the Obama administration tries to work with allies to show Putin he has overplayed his hand, some foreign policy experts on the right are pushing him to take stronger actions against his own studied anti-interventionist tendencies.

While no one in Washington is calling for direct U.S. military intervention in the conflict, some national security experts on the right say it's time to provide military assistance to Ukraine and move U.S. naval warships into the Black Sea to allay concerns in the Balkans about Moscow's aggression.

“It's time to up the ante a bit,” Gary Schmitt, co-director of the Marilyn Ware Center for Security Studies at the American Enterprise Institute, told reporters on a conference call Monday. “The Ukraine needs anti-air capability and anti-tank capability and [help with] intelligence activities.”

Schmitt also said it's time for NATO to mobilize “rapid reaction forces” in coordination with the Balkan states “to make sure there is a line that Russia can't go beyond” and for the U.S. to deploy naval warships to the Black Sea “to remind Russia there is a cost for its activities.”

Military actions should be taken, Schmitt said, along with several economic measures to make it difficult for Russian companies to do business throughout the world, including visa bans on “Russian elites” and freezing Russian bank accounts abroad.

He also suggested it would be worthwhile to go to the United Nations and attempt to pass a resolution condemning Russia's violation of Ukraine's sovereignty even though Moscow would immediately veto it.

“The fundamental policy goal has to be making the Russians see that Putin has overplayed his hand — that the costs of his actions have to be greater than his rewards,” he said.

Other foreign policy experts warn against sending U.S. warships to the region, fearful of reviving the East-West Cold War Imagery of the past and any unintended consequences military-to-military confrontations can produce.

“Military actions can make things worse on the ground without changing the outcome,” said Janine Davidson, a senior fellow for defense policy at the Council on Foreign Relations.

This story was published at 9:32 a.m. and has been updated.