U.S. President Barack Obama is in the Netherlands for an emergency meeting at The Hague with the other leaders of G7 industrialized nations to discuss international responses to the Russian annexation of the Crimean peninsula.
The emergency meeting is taking place on the sidelines of a nuclear security summit. Analysts say the session appears aimed at coordinating Western sanctions imposed on Moscow since Russian troops crossed into Crimea earlier this month. The group also is expected to further evaluate the cost of possible retaliatory measures from Moscow.
Mr. Obama and Secretary of State John Kerry are set to meet with Russian Foreign Minister Sergei Lavrov.
Hours before Mr. Obama's arrival, Russian troops seized control of a Ukrainian military base in Crimea.
Ukrainian soldiers inside the base say Russian troops stormed the compound in the port city of Feodosia using stun grenades and automatic weapons.
In Brussels Sunday, NATO's top commander warned that Russia has deployed a "sizeable" military force on its borders with Ukraine, and says Moscow may be seeking to expand its territorial reach into new areas.
U.S. Air Force General Philip Breedlove said Moscow may be eyeing a small Russian-speaking separatist region on Ukraine's southwestern border known as Transdniester.
Breedlove says the NATO command finds the potential for a Russian move into the Transdniester region, which declared independence from Moldova in the 1990s, "very worrisome."
For its part, Moscow insists it is complying with international troop limits near the Ukrainian border. Additionally, Russia's defense chief sought last week to assure U.S. Defense Secretary Chuck Hagel that Russian troops massed near Ukraine have no intention of crossing the border.
The NATO warning on Transdniester came a day after heavily armed Russian troops fired shots and used armored vehicles to smash through the gates of Ukraine's Belbek air base. Ukrainian forces inside the Crimean base offered no resistance. Russian forces also took control of a Ukrainian naval base at Novofedorovka.
Ukraine has remained highly unstable since November when then-President Viktor Yanukovych backed off of signing a trade agreement with the European Union in favor of closer ties with Russia. The move led to weeks of anti-government protests in Kyiv that forced Mr. Yanukovych to flee the country last month.
Crimeans voted last week in a highly controversial ballot to secede from Ukraine and join the Russian federation -- a move quickly embraced by Russian President Vladimir Putin. The United States and the European Union say the vote is illegal.