Crimeans voted in a referendum on Sunday on whether to break away from Ukraine and join Russia, with Kiev accusing Moscow of pouring forces into the peninsula and warning separatist leaders "the ground will burn under their feet."
Caught in an East-West crisis reminiscent of the Cold War, Kiev said Russia's build-up of forces in the Black Sea region was in "crude violation" of an international treaty, and announced plans to arm and train 20,000 members of a newly-created National Guard to defend the nation.
Western countries say the vote, which is likely to favor union with Russia for a region which has a Russian-speaking majority, is illegal and the United States warned Moscow to expect sanctions.
But Russian President Vladimir Putin said the referendum complied with international law and respected the will of the Crimean people, while his foreign ministry said it had agreed with the United States to seek a solution to the crisis through constitutional reform.
In Kiev, Prime Minister Arseny Yatseniuk threatened dire consequences for Crimean politicians who had called the vote, saying separatist "ringleaders" wanted to destroy Ukrainian independence "under the cover of Russian troops."
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"We will find all of them if it takes one year, two years and bring them to justice and try them in Ukrainian and international courts. The ground will burn under their feet," he told a cabinet meeting.
Yatseniuk had just returned from a U.S. trip where he won expressions of moral support but no offers of weapons. Kiev's pro-European rulers, who took power after last month's fall of Moscow-backed President Viktor Yanukovich to popular unrest, have been as powerless as Western governments to prevent the referendum or buildup of Russian forces on Ukrainian territory.
At a polling booth at a school in Simferopol, the Crimean regional capital, dozens of people lined up outside to cast their ballots early.
"I have voted for Russia," said Svetlana Vasilyeva, 27, a veterinary nurse. "This is what we have been waiting for. We are one family and we want to live with our brothers."
Vasilyeva voiced fears common among some of Ukraine's native Russian-speakers about the consequences of Yanukovich's downfall after protests in which over 100 people were killed. "We want to leave Ukraine because Ukrainians told us that we are people of a lower kind. How can you stay in such a country?" she said.
But ethnic Tatars - Sunni Muslims who make up 12 percent of Crimea's population - said they would boycott the vote despite promises by the regional authorities to give them financial aid and proper land rights.
"This is my land. This is the land of my ancestors. Who asked me if I want it or not? Who asked me?" said Shevkaye Assanova, a Crimean Tatar in her 40s. "For the rest of my life I will be cursing those who brought these people here. I don't recognize this at all. I curse all of them."
Crimea's 1.5 million voters have two options: union with Russia or giving their region, which is controlled by pro-Kremlin politicians, the broad right to determine its own path and choose relations with whom it wants—including Moscow.
Polling stations close at 8 pm. (1800 GMT), with provisional results to be released late on Sunday and the final tally expected a day or two later.
Russia has the right to keep forces on the Black Sea peninsula, including at its naval base in the port of Sevastopol, under a treaty signed after Ukraine gained independence from the wreckage of the Soviet Union in 1991.
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But Ukrainian acting defense minister Ihor Tenyukh accused Moscow of going far beyond an agreed limit on servicemen - which he said was 12,500 for 2014.
"Unfortunately, in a very short period of time, this 12,500 has grown to 22,000. This is a crude violation of the bilateral agreements and is proof that Russia has unlawfully brought its troops onto the territory of Crimea," he said.
This figure had risen from 18,400 on Friday. "Let me say once again that this is our land and we will not be leaving it," he told Interfax news agency.
Tenyukh later said that the defense ministries in Kiev and Moscow had declared a truce until March 21 during which Russian forces, who have been arriving by boat and helicopter, would leave Ukrainian military facilities untouched.