Ukraine's military scrambled aircraft and paratroops to confront Russian troops landing on a spit of land between Crimea and the mainland, defence officials said and the foreign ministry demanded their immediate withdrawal.
The border guard service said Ukrainian forces had taken up defensive positions on Arbatskaya Strelka, running parallel to the east of Crimea, now controlled by Russian forces. It said about 60 Russian troops had landed on the strip and begun digging in, assisted by three armoured personnel carriers.
Six Russian helicopters arrived with 60 more servicemen about two hours later outside the village of Strelkovoye. The border guard service said talks between the two sides established that the Russian servicemen were "guarding against possible terrorist acts" against a gas pumping station.
"At this time, there is no threat of confrontation," the Ukrainian border guard service said.
The defence ministry had earlier said troops dispatched to Arbatskaya Strelka had "immediately" repelled the incursion.
Ukraine's foreign ministry said the village had been occupied by Russian forces, though the border guards gave a different version. "The ministry is making public the Russian incursion and demands the immediate withdrawal of Russian armed forces from Ukraine's territory," it said.
Crimea is home to Russia's Black Sea Fleet under a post-Soviet deal signed with Ukraine. The peninsula has been under the control of Russian forces for two weeks.
Two main roads link Crimea to the mainland, at Armyansk in the west and Chonhar in the east. However, there is a third, minor road connection from the 120 km (75 mile) long Arbatskaya Strelka. Strelkovoye is 30 km (20 miles) south of the town of Henichesk on the Ukrainian mainland.