The Ukrainian Ministry of Foreign Affairs alleged 80 Russian forces invaded a village just north of Crimea, a day ahead of a referendum there to secede from Ukraine.
The ministry claimed Russian forces entered the village of Strilkove, just east of the tip of the Crimean Peninsula and six miles north of the border, with four helicopter gunships and three armored vehicles. It called for the troops to immediately withdraw.
"Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia," the ministry said in a statement.
The move appeared to be the first action by Russian forces outside Crimea, the southern peninsula of Ukraine, where Russian forces moved in late last month. Ukraine, the United States and other allies have called Russia's move into Crimea a violation of international law.
The foreign ministry added: "Ukraine Foreign Ministry declares the military invasion by Russia and demands the Russian side to immediately withdraw its military forces from the territory of Ukraine."
There did not appear to be any gunfire, the newswire reported. Russian Foreign Minister Sergey Lavrov, on Friday, insisted Russia had no intention of moving further into eastern Ukraine.
Russian troops have been camped in Crimea since invading late last month. The region, populated by ethnic Russians, is holding a vote Sunday to decide whether to gain independence from Ukraine and join Russia.