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.
Pro-Russian soldiers walk past trucks that define the limits of their staging area in the vicinity of a Ukrainian military base in Perevalne, Ukraine. (AP photo)
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."
Read: Crimea braces for referendum; Russia vetoes UN resolution, China abstains
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."
Pro-Russian soldiers stand close to the main gate of an Ukrainian military base in Perevalne, Ukraine. (AP photo)
"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.
Read: All you need to know about Crimea's weekend referendum
Russia isolated at UN
Ukraine's report of an invasion came on the second successive day of bloodshed that has 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 Russian activist and a passerby died in a gunfight between a group of Ukrainian nationalists and a Kremlin supporter in the eastern industrial city.
Pro-Russian activists hold the flags of Russia, Kazakhstan and Belarus during a gathering in Sevastopol. (AFP photo)
That incident and another death in the Russian-speaking city of Donetsk on Thursday prompted Russia -- its forces already conducting snap drills on Ukraine's doorstep -- to report "receiving many requests to protect peaceful citizens" in the neighbouring country.
"These requests will be considered," the Russian foreign ministry said.
Yet the seizure of Crimea and ominous threats against the rest of Ukraine left Russia 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 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."
Read: 50,000 protest Kremlin's swoop on Crimea
The measure was backed by 13 of the Security Council's 15 members and saw China abstain -- a massive blow that could shake the Kremlin's confidence in the face of its deteriorating relations with the West.
Kiev said "Russia has isolated itself not only at the UN Security Council, but also from the rest of the world."
Anti-Russian activists stand outside Downing Street during a pro-Ukraine protest in London. (AFP photo)
No common vision
The rugged diamond-shaped Crimean peninsula that has been home to 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 Crimean referendum comes in direct response to three months of deadly protests that toppled Ukraine's 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.
Moscow continues to back Sunday's ballot despite seeing talks of a free travel agreement with Europe it had long sought suspended and facing a painful round of punitive sanctions against top officials that Washington and EU nations are set to unveil on Monday.
Read: Russian troops engage in war games near Ukraine
The worst standoff between Moscow and the West since the 1989 fall of the Berlin Wall found no solution when Kerry and Lavrov locked horns in six hours of talks in London about Crimea, which that ended in a handshake and an agreement that the two sides still remained far apart.
"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 is set to 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.