Around 30 Russian marines surrounded the coast guard base in the Crimean city of Sevastopol on Friday, according to a statement by the Ukraine's state border guard.
The statement, reported by The Associated Press, said the marines were paratroopers from the 810th brigade of Russia's Black Sea fleet and quoted them as saying they were there to prevent extremists seizing the base's weapons.
The Russian defense ministry in Moscow did not comment, according to the AP.
Moscow announced military drills along its border with Ukraine this week. Amateur video surfaced Friday of military helicopters flying into Crimea.
The southern Ukrainian region of Crimea is primarily pro-Russian. It has become the focal point of resistance to the popular uprising that brought about regime change in Kiev last week.
The Associated Press contributed to this report.