US President Barack Obama arrived in the Netherlands on Monday to discuss with six other world leaders how to punish Russia for annexing Ukraine's Crimea region, including possibly excluding Moscow from the G8 club of rich nations.
Obama has called an emergency Group of Seven summit in The Hague to discuss what steps to take against Russia over Crimea, with Russian troops on Monday morning seizing another Ukrainian military base on the peninsula.
Paratroopers and armoured personnel carriers stormed the naval base in Feodosia in the early hours, with vehicles seen leaving the base carrying Ukrainian marines whose hands had been tied.
Russia's near-complete takeover of the Crimea, which it views as a reunification, has forced Western leaders to rethink their relationship with Moscow after a post-Cold War period in which they sought to usher Russia into the broader international community.
With Russia massing what NATO called a "very sizeable" force on its border with Ukraine, there are fears that President Vladimir Putin is hungry for more Ukrainian territory.
The growing crisis is expected to dominate a meeting originally set up to discuss nuclear security.
On Monday, Russian foreign minister Sergei Lavrov is to meet US Secretary of State John Kerry on the sidelines of the Nuclear Security Summit for what may be their most tense talks to date.
It will be their first meeting since Washington imposed financial restrictions on the most powerful members of Putin's inner circle for their decision to resort to force in response to last month's fall of Ukraine's pro-Kremlin regime.
'No more G8'
Kerry has already warned that Moscow risks losing its coveted place among the G8 over its deployment of troops in Crimea.
British Prime Minister David Cameron said leaders from Canada, France, Germany, Italy, Japan and the United States -- minus current G8 chairman Russia -- must discuss the permanent expulsion of Russia from the group, to which it was admitted in 1998 as its reward for choosing a democratic post-Soviet course.
German Chancellor Angela Merkel said last week the political conditions were not in place for a G8 to exist, although her Foreign Minister Frank-Walter Steinmeier later said she had been referring to the June G8 summit in Russia.
Ukraine's interim premier, Arseniy Yatsenyuk, said Sunday he had cancelled plans to be in The Hague so he could hold talks with the IMF on a support programme for his crisis-hit country.
Ukraine's Western-backed leaders also voiced fears of an imminent Russian invasion of the eastern industrial heartland, three weeks after the Kremlin sent troops into the heavily Russified peninsula before sealing its annexation Friday.
Mutual defence obligation
Some other former Communist bloc nations fear for their security in the face of Russian expansionism, and Obama reiterated NATO's solemn obligation to mutual defence.
"No one should ever question the commitment of the United States to the security of Europe," he told the Dutch Volkskrant newspaper on Monday, referring to NATO as "the strongest and most effective alliance in human history."
He noted that sanctions on the Russian economy would also impact the global economy.
"And if Russia continues to escalate the situation, we need to be prepared to impose a greater cost."
Dutch Prime Minister Mark Rutte insisted on Sunday that the G7 meeting would not detract from the aims of the two-day Nuclear Security Summit (NSS), with the G7 talks to wind up on Monday evening.
Leaders of 53 countries are gathering for the third bi-annual NSS, a brainchild of Obama, aimed at preventing a terrorist nuclear attack and reducing the world's non-military nuclear materials.
World leaders should be free on Tuesday to discuss securing the world's stocks of nuclear material to prevent a group like al Qaeda acquiring a nuclear or so-called 'dirty' bomb of conventional explosives wrapped in radioactive material.
Nuclear security is central to Obama's political legacy and in 2009 he called nuclear proliferation "one of the greatest threats to international security".