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.
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.
"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.