The world’s major industrial nations on Monday effectively suspended Russia indefinitely from the Group of Eight and warned that they would impose stronger economic sanctions against Moscow if President Vladi­mir Putin expands his military intervention in Ukraine.

The decision followed a push by President Obama for a united stand by wealthy nations against what he has called Russia’s violation of international law with the annexation of Crimea this month. Obama and the leaders of six allied nations — Canada, France, Germany, Italy, Japan and Britain — agreed Monday to boycott a planned G-8 summit meeting in Sochi, Russia, in June, effectively isolating Putin. Instead, they said they would convene as the Group of Seven in Brussels during the same time frame.

“As long as it is flagrantly violating international law and the order the G-7 has helped build since the end of the Cold War, there is no reason to engage with Russia,” said Ben Rhodes, Obama’s deputy national security adviser for strategic communications. “What Russia has done has been a violation of that entire international order built up over many decades.”

But Russia dismissed the move as unimportant. Russian Foreign Minister Sergei Lavrov, here representing Putin at the Nuclear Security Summit, told reporters that “if our Western partners believe the format has exhausted itself, we don’t cling to this format.”

“We don’t believe it will be a big problem if it doesn’t convene,” Lavrov said. “The G-8 is an informal club. No one hands out membership cards, and no one can be kicked out of it.”

Statement of allied leaders

In a joint statement issued after their meeting Monday, the allied leaders condemned as “illegal” a March 16 Crimean referendum in which residents of the pro-Russian region voted overwhelmingly in favor of joining the Russian Federation. “We also strongly condemn Russia’s illegal attempt to annex Crimea in contravention of international law and specific international obligations,” the statement said. “We do not recognize either.”

Calling Russia’s actions “a serious challenge to the rule of law around the world,” the seven leaders agreed: “Under these circumstances, we will not participate in the planned Sochi Summit. We will suspend our participation in the G-8 until Russia changes course and the environment comes back to where the G-8 is able to have a meaningful discussion.”

They warned that they “remain ready to intensify actions . . . if Russia continues to escalate this situation.”

The meeting on the sidelines of the Nuclear Security Summit was the first of several sessions that Obama is attending with European allies and others over how to persuade Putin, once interested in further integrating Russia into the global economy, to pull back forces from eastern Ukraine and begin a dialogue with the Kiev government to resolve the crisis in Crimea.

But Obama’s three-country tour of Europe, where longtime concerns over Russia’s ambitions beyond Crimea are growing, began against the backdrop of Putin’s public defiance of Western sanctions and the warning of broader ones to come.

On Monday, Ukrainian leaders ordered their forces to leave Crimea under threat from Russian troops, a move U.S. officials here said was a responsible one given the situation’s volatility. Russia’s military consolidation on the Black Sea peninsula follows Putin’s formal annexation of it, a move not recognized internationally.

With Crimea fully in Russian hands, each side has looked toward economic and diplomatic tools to pressure the other.

In his talks here and with European Union officials Wednesday in Brussels, Obama is working to develop a package of economic and financial support for Ukraine’s new pro-Western government, which took power in February after a protest movement ousted pro-Russian president Viktor Yanukovych.

A bipartisan proposal to provide more than $1 billion in aid to Ukraine survived a procedural vote in the U.S. Senate on Monday evening, setting it up for final passage this week. The proposed package would provide $1 billion in loan guarantees for Ukraine and impose sanctions on Russian and Ukrainian officials involved in starting the standoff between the two countries.

Meanwhile, Russian Prime Minister Dmitry Medvedev said pensions for Crimeans would be raised to Russian levels, a decision that will cost the Russian treasury about $1 billion this year. He also said Crimea may be turned into a special economic zone where enterprises would be exempt from taxes until 2015.

The starkest warning about further Russian ambitions came from Ukrainian Foreign Minister Andriy Deshchytsya, who said during an appearance on ABC’s “This Week With George Stephanopoulos” that the prospect of war with Russia is growing.

“We don’t know what Putin has in his mind and what would be his decision,” Deshchytsya said. “That’s why this situation is becoming even more explosive than it used to be a week ago.”

U.S. officials accompanying Obama warned that any additional Russian moves into eastern Ukraine or Moldova’s separatist Transnistria region would represent “a dangerous escalation” of the crisis.

Promoting sanctions

Obama used the nuclear forum, which he inaugurated four years ago in Washington, to rally support for stronger sanctions against Russia in his bid to isolate the country economically and politically in response to its move into Ukraine.

European officials have worried about potential repercussions from Putin should they tighten sanctions in a way that targets specific segments of the Russian economy, including its energy and banking sectors.

Several European nations import more than 90 percent of their energy from Russia, which has used natural gas exports to apply political pressure in the past.

Obama thinks that only extending sanctions beyond individuals close to the Russian leader can force talks toward a political solution in Crimea and dissuade Putin from moving deeper into Ukraine.

“If Russia continues to escalate the situation, we need to be prepared to impose a greater cost,” he said in an interview with Dutch newspaper de Volkskrant ahead of the trip.

Before his evening session with G-7 allies, Obama met with Chinese President Xi Jinping, who is attending the nuclear summit. China is the only veto-holding member of the U.N. Security Council to abstain from voting on a recent resolution condemning Putin’s annexation of Crimea.

U.S. officials said Obama made the case to Xi that he should remain neutral or support sanctions against Russia, given independence sentiments in Tibet and China’s interest in discouraging the kind of popular vote that took place in Crimea.

“Russia cares about its standing in the world,” Rhodes said. “And it matters if traditional friends cannot express support for its actions.”

Leaders of the G-7 convened here at the official residence of Dutch Prime Minister Mark Rutte, who then withdrew because he is not a member of the grouping.

The G-7, formed to coordinate policy among the world’s major free-market democracies, was expanded to include Russia in the late 1990s after the collapse of the Soviet Union and the start of market reforms.

Putin did not attend the nuclear summit, sending Lavrov in his place. Lavrov and Secretary of State John F. Kerry met for an hour Monday, and in one potentially hopeful sign, Lavrov also met with Deshchytsya, the Ukrainian foreign minister.

Russian officials had refused to meet with officials of the new Ukrainian government, calling it illegitimate for taking power in what they call a coup. Whether the meeting could help de-escalate an increasingly dangerous situation was not immediately clear.

Military moves

On Monday, Ukrainian Interim President Oleksandr Turchynov said in the national parliament that he had told the Defense Ministry to issue withdrawal orders for any remaining military personnel in Crimea, where government installations have steadily fallen into Russian hands.

Russian troops control the entire Ukrainian marine base at Feodosia, a Defense Ministry official said, and two senior commanders have been taken into custody.

Russian troops stormed the facility, home to a well-trained and armed force, in the early morning hours, firing automatic weapons and using stun grenades, said Vladislav Seleznyov, a Defense Ministry spokesman. Some of the Ukrainian marines had their hands bound, he said, and 80 to 100 had been gathered in one spot.

Seleznyov said the Russians flew commander Dmitri Deliatizkii and his deputy, Rostilav Lomtev, off the base in a helicopter. Service members in Crimea said Monday afternoon that the orders had not yet reached them.

The commander of a Ukrainian base in Belbek, Yuli Mamchur, has been held by Russians since his base was stormed Saturday. Ukrainian television has reported that he is thought to be held in a jail in the port city of Sevastopol.

“We are military people, and we act on orders,” Seleznyov said by telephone. “When we get orders, we will know where we go and what we’ll take with us.”

Morello reported from Simferopol, Crimea. Will Englund in Moscow and Karen DeYoung, William Branigin and Ed O’Keefe in Washington contributed to this report.