(CNN) -- Ukrainian officials say Aleksandr Vitko, the Russian commander of the Black Sea Fleet, went aboard a blocked Ukrainian warship in Crimea's Sevastopol Harbor on Monday and issued a threat:
"Swear allegiance to the new Crimean authorities, or surrender, or face an attack," he said, according to Vladislav Seleznev, a Ukrainian Defense Ministry spokesman in Crimea who spoke to CNN. He said the Russian commander did not mention an ultimatum deadline.
But a spokesman for the Russian Black Sea Fleet said there are no plans to storm Ukrainian military units in Crimea, according to the Interfax news agency.
The European Union foreign affairs council strongly condemned what it called Russia's "clear violation of Ukrainian sovereignty and territorial integrity" and demanded that Russia pull its forces back to their barracks.
European Union foreign policy chief Catherine Ashton on Monday said the organization will consider "targeted sanctions" against Russia, and U.S. Vice President Joe Biden warned Russian Prime Minister Dmitry Medvedev of increasing political and economic isolation if Russia doesn't move to resolve the situation.
Moscow has defended its parliament's approval of President Vladimir Putin's use of military force to protect its citizens in the Crimean Peninsula, an autonomous region of eastern Ukraine with strong loyalty to Russia.
"I repeat: This is a matter of defending our citizens and our compatriots, of defending the most important human right -- the right to life," Russian Foreign Minister Sergey Lavrov said at a U.N. human rights meeting in Geneva, Switzerland.
But Ukraine's ambassador to the U.N. says Russia's reasoning for a possible invasion is fake.
"There is no evidence that the Russian ethnic population or Russian-speaking population is under threat," Ambassador Yuri Sergeyev told CNN.
The Russian parliament, or Duma, is also considering a law that would allow for the annexation of Crimea, according to the parliament's website.
"Now they are trying to create new legal basis to prove annexation of the territory they're now occupying," Sergeyev said.
A senior U.S. administration official told CNN that Russian forces "have complete operational control of the Crimean Peninsula." The official said the U.S. estimates there are 6,000 Russian ground and naval forces in the region.
"There is no question that they are in an occupation position -- flying in reinforcements and settling in," another senior administration official said.
In Kiev, interim Prime Minister Arseniy Yatsenyuk, who has accused Moscow of declaring war, vowed that his West-leaning government would not give up the region.
"Nobody will give Crimea away. ... There are no grounds for the use of force against civilians and Ukrainians, and for the entry of the Russian military contingent," he said. "Russia never had any grounds and never will."
Ukraine's shaky new government has mobilized troops and called up military reservists.
Camouflaged and unidentified
Former Ukrainian Prime Minister Yulia Tymoshenko, released from jail last week, asked the international community for help.
"I am asking all the world, personally every world leader, to use all the possibilities in order to avoid Ukraine losing Crimea," she told CNN's Christiane Amanpour in an interview.
Ukrainian border guards on Monday reported a buildup of armored vehicles on the Russian side of a narrow sea channel dividing Russia and Crimea, Reuters reported, citing a border guard spokesman.
He said that Russian ships had been moving in and around the city of Sevastopol, where the Russian Black Sea Fleet has a base, and Russian forces had blocked mobile telephone service in some areas. The buildup of Russian armor was near a ferry port on the Russian side of the Kerch Channel, opposite the Ukrainian city of Kerch.
Meanwhile, the Ukrainian State Border Security Service said there had been several attacks on border posts in eastern Crimea just along the border with Russia.
Also on Sunday night, unidentified armed men tried to enter the arms depot in Belbek military base near Sevastopol, said Seleznev, the Ukrainian Defense Ministry spokesman. Ukrainian forces shot into the air to warn them off, but the unidentified men used sound grenades and a Ukrainian commander was wounded as a result, Seleznev told CNN.
The attackers gained access to the base, but Ukrainian troops retain control of the weapons depot and administration building, Seleznev added.
Men dressed in both civilian and camouflage gear with red armbands have patrolled the streets of the regional capital, Simferopol. The area has seen several pro-Russia demonstrations with crowds waving Russian flags and shouting "Thank you, Putin."
Navy defector seeks allies
Ten Ukrainian military and naval bases in Crimea are currently blocked by armed men, the newly appointed naval commander of Ukraine, Rear Admiral Serhei Gayduk, told a Ukrainian TV station.
His predecessor, Denis Berezovsky, who on Sunday said he would not submit to orders from Kiev and defected, was said to have entered the Ukrainian naval base in Crimea under the protection of a group of Cossacks and tried to convince other Ukrainian officers to defect. However, Gayduk was at the base and urged officers to maintain their allegiance to Ukraine, the Defense Ministry's Seleznev told CNN. Troops responded by singing the Ukrainian national anthem.
These scenes come after Putin secured permission from his parliament Saturday to use military force to protect Russian citizens in Ukraine and told U.S. President Barack Obama he had the right to defend Russian interests and nationals, spurning Western pleas not to intervene.
The tensions have worried the West, and Russia's G8 partners have condemned Moscow's military buildup in Crimea. The world's seven major industrialized powers also suspended preparations for the G8 summit in Sochi, Russia, in June.
Their finance ministers announced some economic support for cash-strapped Ukraine.
"We are also committed to mobilize rapid technical assistance to support Ukraine in addressing its macroeconomic, regulatory, and anti-corruption challenges," G7 finance ministers said in a written statement.
U.S. Secretary of State John Kerry, due in Kiev Tuesday, said several foreign powers are looking at economic consequences if Russia does not withdraw its forces.
"It is now of the utmost importance to install calm and de-escalate tensions immediately through dialogue," U.N. Secretary-General Ban Ki-moon told reporters.
Scheduled to meet Lavrov later Monday, he urged "that the Russian Federation refrain from any acts and rhetoric that can further escalate the situation and instead engage constructively and through peaceful means with Ukraine."
German Chancellor Angela Merkel's office said Putin had accepted a proposal to establish a "fact-finding mission" to Ukraine, possibly under the leadership of the Organization for Security and Co-operation in Europe, and to start a political dialogue.
East vs. West
Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been in chaos since Yanukovych was ousted on February 22 after bloody street protests that left dozens dead and hundreds wounded.
Anti-government demonstrations started in late November, when Yanukovych spurned a deal with the EU, favoring closer ties with Moscow instead.
Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.
Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.
Ukrainian leaders and commentators have compared events in Crimea to what happened in Georgia in 2008. Then, cross-border tensions with Russia exploded into a five-day conflict that saw Russian tanks and troops pour into the breakaway territories of South Ossetia and Abkhazia, as well as Georgian cities. Russia and Georgia each blamed the other for starting the conflict.
The crisis has hit Russian stock markets, with Moscow's main MICEX index opening about 8% lower Monday. The central bank hiked interest rates to 7% from 5.5%