Amid rising tensions, the Ukrainian region of Crimea is holding a controversial referendum on March 16 on whether to join Russia.
On the eve of the ballot, Russia vetoed a UN Security Council resolution declaring the referendum to secede from Ukraine invalid.
Thirteen nations voted in favor and Moscow ally China abstained.
U.S. ambassador Samantha Power called it a "sad and remarkable moment."
"Russia cannot change the fact that moving forward in blatant defiance of the international rules of the road will have consequence, nor can it change Crimea's status. Crimea is part of Ukraine today, it will be part of Ukraine tomorrow, it will be part of Ukraine next week, it will be part of Ukraine unless and until its status is changed in accordance with Ukrainian and international law," Power said.
The referendum is expected to pass in Crimea where Russians make up about 60 percent of the population. Ethnic Tatars and many Ukrainians are expected to boycott the vote.
The referendum ballot will feature two questions: One, to grant Crimea greater autonomy within Ukraine. The other -- which is expected to garner the most support -- envisions annexation by Russia.
The United States and European Union have already vowed to take tough action if Russia acts on the results of the referendum.
Russian Foreign Minister Sergei Lavrov has said Moscow will "respect the will of the Crimean people."
Ukraine's government has said Crimea cannot survive on its own -- especially since it depends on electricity, energy and water supplies from the mainland.
Crimea was part of Russia until 1954.
Elsewhere, Ukraine said Russian troops backed by helicopter gunships and armored vehicles had taken control of a village just outside of Crimea.
The action in Strilkove appeared to be the first move outside Crimea, where thousands of Russian forces have been in effective control since late last month.
There were no reports of gunfire or injuries.
Crimea's separatist government said the forces in Strilkove were its own "self-defense forces" sent to defend a gas pumping station.
In Kyiv, visiting U.S. Senator John McCain had tough words for Moscow.
"We are deeply concerned about reports of Russian military units moving closer and in a more tactical way around eastern Ukraine. And I can't say exactly what our response will be, but I think it will be a breach of such enormous consequence that the United States of America and our European allies will be contemplating actions that we never have had before in our relations with Russia," McCain said.
Acting Ukrainian President Oleksandr Turchynov said there was a "real danger" of invasion by Russian troops across Ukraine's eastern border.
He also accused "Kremlin agents" of fomenting unrest in the Russian-speaking east of the country.
Late on March 14, two people were killed and several wounded in a clash in the eastern city of Kharkiv between pro-Russian and pro-Ukrainian demostrators.
On March 13, one pro-Ukrainian protester was killed in the eastern city of Donetsk during clashes with pro-Russian protesters.
In Donetsk on March 15, thousands of pro-Russian protesters stormed the local offices of the national security services.
Russia's Foreign Ministry issued a statement on March 15 claiming Ukrainian nationalist militants were plotting to threaten eastern cities, and that Moscow had received "several requests" to protect civilians.
In Moscow, tens of thousands took to the streets to protest Moscow's actions in Ukraine, while another demonstration supporting them was held near the Kremlin.
In a related development, NATO said several of its websites were brought down in a cyber attack on March 15.
A group calling itself "cyber berkut" said the attack had been carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country.
Cyber berkut is a reference to the feared and since disbanded riot squads used by the government of ousted pro-Russian Ukrainian President Viktor Yanukovych.
As well as the main NATO website www.NATO.int, the website of a NATO-affiliated cyber security centre in Estonia was also affected.
The alliance said none of its essential systems were compromised.
Based on reporting by AP, dpa and Reuters