As Russia ignores international warnings and mounts a full takeover of Ukraine’s strategic region of Crimea, White House officials and U.S. lawmakers struggled Sunday with how to deal with Russian President Vladimir Putin’s defiant aggression.

Secretary of State John F. Kerry promised serious repercussions for the violation of Ukraine’s sovereignty, echoing President Obama’s warning last week about the “costs” of military intervention.

Yet the U.S. appears to have few effective options to punish Russia for its actions, which Ukrainian officials consider to be “a declaration of war.”

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Mr. Kerry, who appeared on four Sunday political talk shows, stressed that all options are on the table, but it’s clear military force is unlikely.

Instead, the U.S. and its allies are considering economic sanctions and a suspension of the nearly $40 billion-per-year U.S.-Russia trade relationship.

In another move of condemnation, the U.S. and its key allies — Canada, France, Germany, Italy, Japan and Britain — have halted planning efforts for the June meeting of the Group of Eight, set for Sochi, Russia.

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But those moves, if they come to fruition, may not sway Russia, some analysts say.

For Mr. Putin, who has invaded Ukraine under the auspices of protecting ethnic Russians and securing Russian military bases in the flashpoint region of Crimea, punishment from the international community may pale in comparison with the benefits of annexing key parts of Ukraine and expanding his influence across the old Soviet bloc.

“He’s made a cost-benefit analysis. He has weighed the costs of doing what he’s done and he has viewed the benefits of it and clearly he has concluded the benefits far outweigh the costs. We need to endeavor to change that calculus,” Sen. Marco Rubio, Florida Republican, said during an interview on NBC’s “Meet the Press” on Sunday.

Ukraine’s new government — formed last week after months of protests aimed at the nation’s former pro-Russian president, Viktor Yanukovych — called up reservists Sunday as Russian troops increase their presence in Crimea. Mr. Putin’s forces already have taken control of several military bases in the area, and hundreds of Russian troops also were seen heading toward Crimea’s regional capital of Simferopol.

Men in military uniforms with no insignias already are in the city’s central plaza, according to news reports.

NATO leaders called a meeting Sunday to discuss the situation. Secretary General Anders Fogh Rasmussen called on Moscow to “de-escalate the tensions” in Crimea.

Mr. Kerry made similar admonitions, saying Russia’s actions call into question the country’s “capacity to be within the G-8.” He suggested that isolation by the U.S. and its allies and the removal of Russia from the powerful G-8 and G-20 organizations could be a key tactic moving forward.

Mr. Kerry also called Mr. Putin’s military aggression “a 19th-century act in the 21st century,” saying Moscow risks irreparable damage to its relationships with other nations.

The secretary of state also announced plans to travel to Kiev on Tuesday and meet with senior representatives of Ukraine’s new government.

President Obama on Sunday spoke via phone with Prime Minister David Cameron of Britain, Polish President Bronislaw Komorowski and German Chancellor Angela Merkel, according to the White House. The leaders discussed a financial aid package to Ukraine, and the administration stressed that working with international allies is vital as it considers the path forward.

“The United States and the president is currently considering all options. They are all on the table,” Mr. Kerry said on ABC’s “This Week” program on Sunday. “There’s no question but that Russia needs to understand this is serious, and we and other friends and allies engaged in this are all deadly serious about this. You cannot behave this way in the 21st century and sit around the table with the normal entities and pretend that life is as usual. It is not going to be as usual.”

Mr. Kerry specifically mentioned “very serious repercussions on trade, on investment, on assets, asset freeze” and other steps.

That slate of options will include U.S.- and European Union-led economic sanctions against Russia. A group of senators also has proposed targeting the U.S.-Russia trade relationship.

The U.S. imported nearly $27 billion in Russian goods last year while selling more than $11 billion in American products to Russia. Cutting off the U.S. as a market for Russian businesses, in conjunction with other economic sanctions, could deal a significant blow to Russia’s economy, supporters of the strategy argue.

But that route may lead to Russian retaliation and, in turn, hurt the U.S. or its allies, said Steven Bucci, director of the Douglas and Sarah Allison Center for Foreign and National Security Policy at the Heritage Foundation.

“Could they retaliate? Yes. They’re probably not going to retaliate directly against the U.S., but they could put pressure on the Europeans. Europe is much more beholden to Russia for energy than we are,” Mr. Bucci said, referencing European nations’ reliance on significant amounts of Russian oil and gas. “Short of sanctimonious comments by Secretary of State Kerry and a few others, I’m not sure we’ll even get that far down the road. I’m really disheartened by this.”

Mr. Bucci also said Russian cyberattacks could be launched against U.S. or European targets as a result of economic sanctions or a severing of trade relationships.

Meanwhile, protesters in Kiev reportedly held signs asking for U.S. help in holding off Russian forces. A resolution that cleared the Russian legislature’s upper house over the weekend opens the way for military force “in the territory of Ukraine,” a sign Mr. Putin may move beyond Crimea.

Seeing the full power of Moscow on his doorstep, Ukraine’s ambassador to the United Nations, Yuriy Sergeyev, appealed to Mr. Putin’s faith.

“If he’s a Christian, if he demonstrates his Christianity, instead of preparing to kill us he should pray for us,” he said.

But with Ukrainians fearing for their lives, even the most hawkish American lawmakers don’t see military intervention as a viable option.

Instead, the White House and its international partners likely will pursue some combination of economic punishment and isolation.

“Russia believes there is nothing going to stop them, which is why they’ve become so aggressive in Crimea. There is not a lot of options on the table and, candidly, and I’m a fairly hawkish guy, sending more naval forces to operate in the Black Sea is not really a very good idea,” said Rep. Mike Rogers, Michigan Republican and chairman of the House Permanent Select Committee on Intelligence, speaking during an interview on “Fox News Sunday.” “Unless you’re intending to use them, I wouldn’t send them. So now you’ve got only economic options through the EU. I would use those.”