THE HAGUE, Netherlands (AP) — Seeking to intensify pressure on Moscow, western powers sought Monday to isolate Russia over its actions in Ukraine, insisting that a planned summit of leading economic powers to be hosted by Vladimir Putin would not go forward.

World leaders gathering in the Netherlands were angling for ways to prove Russia would face increasing estrangement from the powerful Group of Eight world powers unless it changes course in Ukraine. British Prime Minister David Cameron and German Chancellor Angela Merkel said a summit scheduled for June in Sochi was now off the table, while the United States warned that Russia’s global standing would continue to deteriorate as the West sought to present a united front against Putin.

“As long as the political environment for the G-8 is not there, as at the moment, there is no G-8 — neither as a concrete summit nor as a format,” Merkel said.

Leaders of the reconstituted Group of Seven — Russia excluded — huddled in an emergency meeting at the Dutch prime minister’s residence to plot a path forward. President Barack Obama’s deputy national security adviser, Ben Rhodes, said the session was aimed at foreshadowing “what economic sanctions Russia will be faced with if it continues down this course.” He said the countries also would discuss assistance for the fledgling Ukrainian government.

As long as Putin keeps “flagrantly violating international law,” there’s no reason for the G-7 to engage with Russia, Rhodes said. At the same time, he suggested that the US and other nations were not prepared to formally kick Russia out of the G-8.

“The door is open to Russia to deescalate the situation,” Rhodes said.

But British Prime Minister David Cameron said a scheduled G-8 meeting in Sochi would not take place.

“We should be clear there’s not going to be a G8 summit this year in Russia. That’s absolutely clear,” Cameron told British television. “We will be meeting tonight, the seven other countries of the G8, to determine the way forward. But frankly it’s Russia that needs to change course.”

The delicate diplomacy took place on the sidelines of a long-planned nuclear security summit in The Hague, where the official topic — nuclear terrorism — was quickly overshadowed by the West’s alarm over Russia’s move to annex the Crimean Peninsula from Ukraine.

Within hours of arriving Monday morning in the Netherlands, Obama’s first stop on a weeklong international trip, the president declared that the US and Europe stand together behind Ukraine as he met with Dutch Prime Minister Mark Rutte in Amsterdam. “We’re united in imposing a cost on Russia for its actions so far,” Obama said.

Obama also sought to coax support from one of Moscow’s closest allies during one-on-one talks with Chinese President Xi Jinping. China often sides with Russia in disputes with the West, but US officials have been appealing to Beijing’s well-known opposition to outside interference in another nation’s domestic affairs.

Obama treaded carefully in statements with Xi before their meeting, saying only that they would discuss the situation in Ukraine.

“I believe ultimately, that by working together, China and the United States can help strengthen international law and respect for the sovereignty of nations and establish the kind of rules internationally that allow all peoples to thrive,” Obama said in a subtle appeal for Chinese support.

China, a frequent Russian ally, abstained a week ago from voting on a United Nations Security Council resolution declaring Crimea’s secession referendum illegal. With Russia vetoing the measure and the 13 other council members voting in favor, China’s abstention served to isolate Moscow internationally.

In a counterpoint to Obama and his G-7 partners, a group of five major emerging economies — Brazil, Russia, India, China and South Africa — issued a statement Monday opposing sanctions and urging nations to work through the UN instead. The so-called BRICS nations said hostile language, sanctions and force do not “contribute to a sustainable and peaceful solution.”

The long-planned nuclear summit in The Hague kicked off with a highly theatrical opening ceremony, plus an announcement that Japan would turn over to the US more than 700 pounds of weapons-grade plutonium and a supply of highly-enriched uranium, a victory for Obama’s efforts to secure nuclear materials around the world.

But there was little time to celebrate that victory. Instead, the US and Europe were focused squarely on Russia’s annexation of the Crimean Peninsula and the fear that Moscow could decide to expand further into Ukraine.

On Tuesday, Obama plans a joint meeting with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye, a session preceded by a sit-down with Prince Mohamed bin Zayed, crown prince of Abu Dhabi, the richest emirate in the United Arab Emirates federation.

In an interview with the Dutch newspaper de Volkskrant published before he arrived Monday, Obama said his message to European leaders is that Putin needs to “understand the economic and political consequences of his actions in Ukraine.”

Still, he said he does not view Europe as a battleground between the East and the West.

“That’s the kind of thinking that should have ended with the Cold War,” Obama said.

AFP contributed to this report.