Ukraine accused Russia on Saturday of invading a region bordering Crimea and vowed to use "all necessary measures" to ward off an attack that came on the eve of the peninsula's breakaway vote.
The dramatic escalation of the most serious East-West crisis since the Cold War set a tense stage for the referendum on Crimea's secession from Ukraine in favour of Kremlin rule -- a vote denounced by both the international community and Kiev.
The predominantly Russian-speaking Black Sea region of two million people was overrun by Kremlin-backed troops days after the February 22 fall in Kiev of a Moscow-backed regime and the rise of nationalist leaders who favour closer ties with the West.
President Vladimir Putin defended Moscow's decision to flex its military muscle by arguing that ethnic Russians in Ukraine needed "protection" from violent ultranationalists who had been given free reign by the new Kiev administration.
But Russian Foreign Minister Sergei Lavrov had told Secretary of State John Kerry in London on Friday that Moscow "has no, and cannot have, any plans to invade the southeast region of Ukraine."
The invasion reported by the Ukrainian foreign ministry was small in scale and concerned a region that lies just off the northeast coast of Crimea called the Arabat Spit.
The Ukrainian ministry said 80 Russian military personnel had seized a village on the spit called Strilkove with the support of four military helicopters and three armoured personnel carriers.
The Ukrainian "foreign ministry declares the military invasion by Russia and demands the Russian side immediately withdraw its military forces from the territory of Ukraine," it said in a statement,
"Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia."
There was no immediate response to Ukraine's announcement from Moscow but Washington's UN representative Samantha Power called any new Russian troop movement in south Ukraine an "outrageous escalation".
Ukraine's claim of the invasion came on the second successive day of bloodshed that has now killed three people in the heavily Russified southeast of the culturally-splintered nation of 46 million.
The latest deadly violence flared on Friday evening in Kharkiv when a group of nationalists opened fire on pro-Russian supporters in the heart of the eastern industrial city of 1.4 million.
No one was hurt but police said the pro-Russians then chased the gunmen to the headquarters of a far-right group called Patrioty Ukrainy (Ukrainian Patriots).
Police said a pro-Russian protester and a passer-by were killed when the nationalists holed up inside the building opened fire. Six others were hurt -- including one officer -- when police arrived at the scene.
That incident and another death in the Russian-speaking city of Kharkiv on Thursday prompted the Russian Foreign Ministry -- its forces already conducting snap drills on Ukraine's doorstep -- to report "receiving many requests to protect peaceful citizens" in its western neighbour.
"These requests will be considered," the Russian foreign ministry said.
Ukraine's acting President Oleksandr Turchynov gravely told a session of parliament that Moscow's repeated warnings meant "there is now a real danger of a (Russian) invasion on the territory of Ukraine".
Yet Russia's seizure of Crimea and ominous threats against the rest of Ukraine found the nuclear power staring in the face of international isolation when it was abandoned by key geopolitical ally China at a crucial UN Security Council vote on the crisis in New York.
Russia was alone in vetoing a US-drafted UN Security Council resolution reaffirming that the Crimean referendum "can have no validity" and that Ukraine must remain a sovereign state.
"Russia, isolated, alone and wrong, blocked the resolution's passage," Power said. "This is a sad and remarkable moment."
The measure was backed by 13 of the Security Council's 15 members and saw Russia's geopolitical ally China abstain -- a massive blow that could shake the Kremlin's confidence in the face of its deteriorating relations with the West.
The rugged diamond-shaped region that has housed tsarist and Kremlin navies since the 18th century is widely expected to vote in favour of Kremlin rule after its lawmakers declared independence from Kiev earlier this month.
The referendum comes in direct response to three months of deadly protests that toppled the pro-Kremlin president and brought to power a new European-leaning team in Kiev that threatens to shatter Putin's dream of rebuilding a post-Soviet empire.
Kiev has denounced the Crimean vote as illegal but is also warily watching as separatist sentiments spread through other southeastern regions with centuries-old cultural and trade links to Russia.
Yet Moscow backs the ballot despite a new round of painful sanctions against top Russian officials that Washington and EU nations are expected to unveil on Monday.
The worst standoff in East-West relations since the 1989 fall of the Berlin Wall found no solution when Kerry and Lavrov locked horns in six-hour talks in London about Crimea that ended in a handshake and an agreement that the two sides remained as far as before.
"We have no common vision of the situation," Lavrov grimly told reporters.
A US diplomat said Kerry found himself at check-mate when Lavrov "made it clear that President (Vladimir) Putin is not prepared to make any decision regarding Ukraine until after the referendum on Sunday."
That timing is far too late for US officials who accuse Crimea's separatist leaders and their Kremlin backers of holding the vote at "gunpoint".
The European Union will debate travel bans and asset freezes on Monday against Russian officials held responsible for threatening Ukraine's territorial integrity.
Germany's Bild daily cited Western diplomats as saying that the Russians on the joint US-EU travel ban list will include Defence Minister Sergei Shoigu and Kremlin chief of staff Sergei Ivanov along with other top Putin advisers.
US officials have stressed that Putin himself is not on the sanctions list.
The threat of Western economic sanctions saw the Moscow stock market lose 11 percent of its value last week -- a drop that the Wealth-X research firm said cost the country's top 10 tycoons $6.6 billion and may put additional domestic pressure on Putin.