US President Barack Obama's threats of "costs" did not sway Vladimir Putin's calculations on Crimea.

Now he must see if the pain he plans to escalate on Russia will be sufficiently acute to do any better.

The White House's initial strategy of seeking to defuse the crisis, offering Russia an "off-ramp" and warning of consequences has run its unsuccessful course.

Putin's swift absorbtion of Crimea and his fiery speech Tuesday, which seemed to put a full stop on the post-Cold War era, forced the Obama administration into a new phase.

The plan now is to hike up the economic cost to Moscow and punish its move into the Ukrainian region by deepening its isolation.

Washington must also try to keep the crisis from careening dangerously out of control -- all while bolstering new allies in post-Soviet Europe and firming the West's resolve.

In the longer term, the White House faces an uncomfortable self-examination of how it got here. Yet another US look at how to handle President Putin is in order.

The strategic implications meanwhile of the worst East-West crisis since the Cold War threaten to consume much of the political bandwidth of Obama's remaining second term.

Obama will head to Europe next week on a trip which is suddenly the most important by a US president in many years.

He called G7 leaders to meet him in The Hague and will try to lock in Russia's isolation and convince European states to build a sanctions regime with teeth.

"We can calibrate our response based on whether Russia chooses to escalate or to de-escalate the situation," Obama said Monday.

Washington sanctioned 11 officials, including members of Putin's inner political circle, on Monday, drawing only sneers from Moscow.

But the White House promises more to come.

Its economic leverage however over Moscow is finite, despite a widening trade relationship which has seen US firms such as Boeing and Exxon Mobil become players in the Russian market.

Europe, with its huge trade and energy relationships with Moscow, holds the key.

"The Europeans are in a better position to do damage to the Russian economy," said Anton Fedyashin, a Russia expert at American University.

But will European leaders, for all their tough talk, follow through with measures that will also inflict a cost on their own fragile economies?

"I seriously doubt that most European countries are going to say, 'Yes, we are ready to support that plan,'" Fedyashin said.

So far, Washington has targeted individuals and not the wider Russian economy. But it has hinted politically powerful tycoons should beware.

Longer term, it must decide whether to attempt the kind of banking sanctions which humbled Iran's economy.

Even if it does not, US officials hope that keeping such a threat on the table could dampen Russian economic confidence and force Putin's hand.

"I wouldn't, if I were you, invest in Russian equities right now -- unless you are going short," said White House spokesman Jay Carney.

Iron-clad

Russia's incursion into Crimea and the fear of a move further into eastern Ukraine sent a Cold War chill through Europe.

It prompted US Vice President Joe Biden to slip behind the old Iron Curtain with an unequivocal strategic message.

"I want to make it unmistakingly clear to you and to all our allies in the region that our commitment to mutual self-defense under Article 5 of NATO remains iron-clad," Biden told Polish President Bronislaw Komorowski on Tuesday.

Obama will pick up the banner next week and Washington has already sent six extra F-15 fighters to step up NATO air patrols over the Baltic.

The Crimea crisis may also bring NATO, which has struggled to define a post-Cold War role, back to its geostrategic roots. Tough questions are pending about sinking European defense budgets and Obama's own plans to trim Pentagon accounts.

The president's political foes are calling on him to revive plans to station US missile defense interceptors in Poland and the Czech Republic.

But a senior US official effectively scotched the idea, saying missile defense "has never been about Russia."

Bankroll Ukraine

Obama has repeatedly asked Congress to pass $1 billion in loan guarantees for Ukraine.

But officials have been coy on reported Ukrainian requests for military aid to avoid provoking the Kremlin.

Obama will also next week urge allies to do more to help Ukraine navigate an economic crisis exacerbated by the loss of a huge loan from Russia.

The International Monetary Fund and the World Bank will be at the forefront -- but the bill is not cheap: Ukraine needs 25 billion euros over two years.

Even as he cranks up the heat on Putin, Obama will seek to cool military tensions between two powers which control 90 percent of the world's nuclear arms.

A move into eastern Ukraine "would be as egregious as any step that I can think of," Secretary of State John Kerry said in a clear warning to Moscow.

Such a scenario would "require a response that is commensurate with the level of that challenge."

But can anyone really know how seriously Putin now takes American threats?

Three-hour ultimatum

Pro-Russian forces earlier seized two Crimean navy bases and detained Ukraine's naval chief as Moscow tightened its grip on the flashpoint peninsula despite Western warnings that its "annexation" would not go unpunished.

Dozens of despondent Ukrainian soldiers -- one of them in tears -- filed out of Ukraine's main navy headquarters in the historic Black Sea port city of Sevastopol after it was stormed by hundreds of pro-Kremlin protesters and masked Russian troops.

The local prosecutor's office said Ukraine's navy commander Sergiy Gayduk -- appointed after his predecessor switched allegiance in favour of Crimea's pro-Kremlin authorities at the start of the month -- had been detained.

Russian Defense Minister Sergei Shoigu later urged Crimea's pro-Russian leaders to free Gayduk, but only after the expiry of a 9:00 pm (1900 GMT) deadline set by Ukraine's acting president Oleksandr Turchynov for the Crimean authorities to release the commander.

His capture was a blow to efforts by a new team of untested pro-Western leaders in Kiev to impose some authority in the face of an increasingly assertive Kremlin.

Ukraine's National Security and Defense Council chief Andriy Parubiy said Kiev had decided to withdraw from the Moscow-led Commonwealth of Independent States (CIS) alliance that replaced the Soviet Union and to slap visas on Russians who sought to enter the country.

Parubiy added that Ukraine was also developing a contingency plan to withdraw Crimean servicemen and their family members "so that they could be quickly and efficiently moved to mainland Ukraine".

China urges political solution to Crimean issue: UN envoy