The gathering of world leaders, including United States President Barack Obama, at The Hague on Monday is scheduled to consider nuclear security issues. But its opening has been overshadowed by an emergency meeting of the Group of Seven industrialized nations - the US, the UK, Canada, France, Germany, Italy and Japan, excluding Russia - to consider how to punish Moscow for its actions in Crimea.

"Europe and America are united in our support of the Ukrainian government and the Ukrainian people; we're united in imposing a cost on Russia for its actions so far," Obama told journalists during a visit to an Amsterdam museum on Monday.

Russian Foreign Minister Sergei Lavrov is due to meet US Secretary of State John Kerry on the sidelines of the Nuclear Security Summit.

It will be their first meeting since the US imposed financial sanctions on powerful Russians for the country's intervention in Ukraine, in which it responded to February's fall of Ukraine's pro-Kremlin regime by annexing the Crimea peninsula.

Kerry has warned that Moscow was risking losing its place in the G8 because it deployed troops into Crimea. Russian President Vladimir Putin was not due to attend the talks.

There are fears that Putin is eyeing more of Ukraine's territory, with NATO saying Russia had a "very, very sizeable and very, very ready" number of troops on its border with Ukraine.

Military bases taken over

Early on Monday, Russian troops used stun grenades and fired automatic weapons to take over a top Ukrainian marine base in Crimea. Ukrainian soldiers were taken away for questioning.

The storming of the Feodosia naval base in eastern Crimea comes two days after Russian troops used similar tactics to take control of Ukraine's Belbek airbase, also on the Black Sea peninsula. Russia has already claimed several other bases and naval vessels.

The Feodosia base was seen as one of the last remaining symbols of resistance to Russia's takeover of Crimea, which it annexed in a move condemned by Ukraine and the West on March 21, days after a referendum in which Crimea overwhelmingly voted to join Russia.

Ukraine's acting president, Oleksander Turchinov, told the country's parliament on Monday that Ukraine would pull its forces out of Crimea and evacuate the families of military service personnel following "threats" from the Russian military.

Turchinov said the decision was made in the face of "threats to the lives and health of our service personnel and their families."

se/slk (AFP, Reuters, dpa)