THE HAGUE, Netherlands — 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 Barack 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.”

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.