Ukraine's prime minister said Tuesday that the conflict in Crimea has entered a military phase and accused Russia of commiting a "war crime" by firing on Ukrainian servicemen.
A military spokesman said one soldier was killed and another injured. Ukrainian servicemen in Crimea have now been authorized to use their weapons in order to defend their lives.
"The conflict is moving from a political one to a military one because of Russian soldiers," Arseniy Yatsenyuk said at a meeting at Ukraine's defense ministry. "Today, Russian soldiers began shooting at Ukrainian servicemen and this is a war crime without any expiry under a statute of limitations."
The serviceman was shot and killed while manning a tower overlooking a vehicle pool at the base, according to a defense ministry statement. It said the attackers wore Russian military uniforms.
Yatsenyuk has reportedly ordered Ukraine's defense minister to call a meeting with his counterparts from Britain, France, and Russia - signatories to a 1994 agreement guaranteeing Ukraine's borders - to prevent an escalation of the conflict.
The incident came just hours after Russian President Vladimir Putin and Crimean leaders signed a treaty to make the Black Sea peninsula part of the Russian Federation, a move the White House immediately condemned.
"This action...will never be recognized by the United States and the international community,'' spokesman Jay Carney said.
Carney said the administration is preparing to expand sanctions it imposed on Monday along with the European Union.
Russian Foreign Minister Sergei Lavrov told U.S. Secretary of State John Kerry that Western sanctions were unacceptable and would not remain without consequences, the Russian ministry said in a statement Tuesday.
Lavrov and Kerry spoke by telephone after the treaty signing.
President Barack Obama has invited G-7 allies to meet next week to consider further response to the Crimea crisis. The meeting will take place on the sidelines of a nuclear security summit at The Hague that Obama plans to attend.
The Russian parliament is expected to begin the process of ratifying the treaty within days, the Itar-Tass news agency cited a senior lawmaker as saying.
Ukraine's Foreign Ministry said Tuesday that it does not recognize the treaty.
A pro-Russian crowd watches a live broadcast of Russian President Vladimir Putin's speech on Crimea, in Sevastopol, Crimea, Ukraine, March 18, 2014.
City council workers clear a barricade on a road leading to Kyiv's Independence Square, Ukraine, March 18, 2014.
An elderly woman holds a calendar depicting Soviet leader Josef Stalin while watching a broadcast of Russian President Vladimir Putin's speech on Crimea, as thousands of pro-Russian people gathered to watch the address, in Sevastopol, Crimea, Ukraine, March 18, 2014.
Russia's President Vladimir Putin addresses the Federation Council in Moscow's Kremlin, Russia, March 18, 2014.
Police look at portraits of missing political activists and journalists that protesters pasted on the gate of the Crimean Interior Ministry in Simferopol, March 18, 2014.
Members of a "Maidan" self-defense battalion take part in a training exercise at a Ukrainian Interior Ministry base near Kyiv, March 17, 2014.
A Ukrainian serviceman guards a checkpoint near the village of Strelkovo in the Kherson region adjacent to Crimea, March 17, 2014.
Members of a Crimean self-defense unit speak with a motorcyclist waving a Russian flag in Simferopol, Crimea, Ukraine, March 17, 2014.
Armed men, believed to be Russian, dig trenches near the Ukrainian military base in Perevalnoye outside Simferopol, March 17, 2014.
A pro-Russian crowd celebrates in the central square in Sevastopol, Ukraine, March 16, 2014.
People wrapped in Russian flags watch fireworks during celebrations after the preliminary referendum results were announced in Lenin Square in the Crimean capital Simferopol, March 16, 2014.
A woman casts her ballot at a polling station during the Crimean referendum, in Sevastopol, Ukraine, March 16, 2014.
In a referendum Sunday, widely believed to have been orchestrated by Moscow, Crimean voters backed the peninsula's secession from Ukraine. The U.S. and the European Union declared the vote illegal and in violation of Ukrainian and international law.
Crimean officials said the final ballot count showed 97 percent of voters favoring independence from Ukraine.
However, senior White House officials told reporters they have concrete evidence that some ballots in the referendum were pre-marked when they arrived in cities before the vote.
US, NATO allies condemn Russia
Vice President Joe Biden called Russia's annexation of Crimea a "land grab" and said Washington is committed to defending the security of its NATO allies on Russian borders.
Biden flew from Poland to Lithuania on Tuesday after meeting with Polish leaders and the leader of Estonia. Tomorrow he will meet with the presidents of Lithuania and Latvia.
Biden said the U.S. is considering sending troops for war games in the Baltic states bordering Russia, in a move aimed at reassuring NATO allies alarmed by Moscow's actions regarding Crimea.
Separately, NATO Secretary-General Anders Fogh Rasmussen said Russia was going down a "dangerous path" by annexing Crimea.
He said on Tuesday Russia has disregarded all calls to step back into line with international law. Fogh Rasmussen stressed that no NATO ally will recognize what he called an illegal and illegitimate action.
In Britain, Foreign Secretary William Hague also condemned Russia's actions.
"The crisis in Ukraine is the most serious test of European security in the 21st century so far," Hague said.
The White House says President Barack Obama spoke by phone Tuesday with German Chancellor Angela Merkel and agreed on the need to immediately send international monitors to southern and eastern Ukraine.
US Navy runs Black Sea exercise
The Truxtun, a U.S. guided-missile destroyer, started a one-day military exercise with the Bulgarian and Romanian navies in the Black Sea on Wednesday, a U.S. Naval Forces official told Reuters on Wednesday.
Ukraine's Crimea peninsula juts into the north of the Black Sea. The U.S. military has described it as a “routine” deployment scheduled well before the crisis in Ukraine.
“There are many reasons for exercises with allies, it allows us an opportunity to assure our NATO allies that we support them,” Shawn Eklund, a public affairs officer for U.S. Naval Forces Europe, told Reuters.
Calling Kyiv the cradle of Russian civilization, Putin expressed hope Russia and Ukraine can continue to co-exist.
US Freezes Russians' Assets As Ukraine Crisis Deepens
But with reports of several incursions by Russian or Russian-backed armed personnel in eastern Ukraine, outside of Crimea, there is rising concern throughout the country whether Russia will be satisfied with only annexing Crimea.
Ukraine's Prime Minister Yatsenyuk says there is "convincing evidence" Russian special services are organizing unrest in the eastern part of the country.
"There are saboteurs who have been arrested," Yatsenyuk said. "There is no place in Ukraine for these warmongers."
Some Ukrainians tell VOA their families, even in the central part of the country, are stocking up on bread, water and medications, due to concerns tensions will escalate in the next several months amid worries there could be war.
Putin says Moscow has no designs on other parts of the former Soviet republic.
In 1954, then Soviet leader Nikita Khrushchev gifted the Crimean peninsula to Ukraine, then part of the USSR.
Ukraine not seeking NATO membership
Ukraine's new pro-Western leadership is not seeking membership in NATO, Prime Minister Yatsenyuk said on Tuesday, in comments intended to reassure Russia and Ukraine's large number of Russian-speakers.
"Strictly with a view to maintaining Ukraine's unity, the question of joining NATO is not on the agenda,'' Yatsenyuk, who normally speaks in Ukrainian, said in a 10-minute televised appeal delivered in Russian. "The country will be defended by a strong, modern Ukrainian army.''
Yatsenyuk also said decentralization of power was a key plank of government policy, adding that Kyiv's efforts to integrate with Europe would take into account the interests of Ukraine's mainly Russian-speaking industrial east.
Ukrainians, from various cities of the country, reacted to Crimea’s annexation by Russia with outrage, some convinced it will only be temporary, others voicing concern that it represented only the beginning of Putin’s ambitions.
“[The Russians] will show off for a little while and then we will get Crimea back…. It will be Ukrainian. And the Crimeans - even those who are advocating for Russia - in a year or two will understand that life in Ukraine was much better,” said Yuriy Zborovsky from the central Ukrainian city of Kirovograd.
“I want [Vladimir Putin] to stop mocking our people. I want him to stay in Russia and govern [the Russians] and not touch our people and not torture Ukraine,” said Kyiv resident Lyubov Semenyaka.
“Without doubts it will not end here; Putin will continue to step over Ukraine because Crimea is not his ultimate goal. His ultimate goal is to invade all of Ukraine,” said Vasyl Pazenyak from the western city of Lviv.
VOA's Steve Herman contributed to this report from Kyiv; some reporting by Reuters.