A Ukrainian anti-aircraft battery fired warning shots to prevent an incursion on Saturday by Russian helicopter gunships from the Crimea peninsula, the Ukrainian defence ministry said.
Two Mi-35 aircraft approached the boundary between mainland Ukraine and Crimea, occupied and annexed by Russia two months ago, at 12:55pm (0955 GMT), it said in a statement.
Troops stationed on Kutara Point "fired warning shots from an anti-aircraft system, forcing the provocateurs to turn back".
The incident happened on the eve of a Ukrainian presidential election that the Kiev authorities hope will strengthen their position against Moscow, despite difficulties holding the ballot in eastern areas controlled by pro-Russian separatist rebels.
Since the Russian takeover of Crimea in March there have not been major incidents between Ukrainian and Russian troops, either around Crimea or on Ukraine's eastern borders where Russia has built up its forces close to the frontier.
Dozens of people have been killed in the east in recent weeks, however, in clashes between Ukrainian security forces and militants who reject the authority of the leadership in Kiev.