KIEV - Ukraine accused Russia on Saturday of invading a region bordering Crimea and vowed to use "all necessary measures" to repel an attack that came on the eve of the Black Sea peninsula's breakaway vote.
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 dramatic escalation of the most serious East-West crisis since the Cold War set a tense stage for Sunday's 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 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 has defended Moscow's decision to flex its military muscle arguing that ethnic Russians in Ukraine needed "protection" from violent ultranationalists - even though Russian Foreign Minister Sergei Lavrov told US Secretary of State John Kerry on Friday that Moscow had no plans "to invade the southeast region of Ukraine."
But the Ukrainian foreign ministry said 80 Russian military personnel had seized a village on the Arabat Spit called Strilkove with the support of four military helicopters and three armoured personnel carriers.
The ministry in a statement demanded that "the Russian side immediately withdraw its military forces from the territory of Ukraine."
"Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia."
Footage released on YouTube that claims to have been shot near Strilkove showed two attack helicopters circling at low altitudes around a highway that cuts through empty fields.
There was no immediate response to Kiev's invasion announcement from Moscow, but Washington's UN ambassador Samantha Power called any new Russian troop movement in south Ukraine an "outrageous escalation".
The peninsula's pro-Kremlin administration later released a statement saying Strilkove had been "taken under the control of self-defence forces of Crimea". They were to protect a natural gas pumping station that had allegedly come under attack from a group of Ukrainian nationalists.