Ukraine on Thursday imposed harsher restrictions on the entry of Russian male adults in the country over a potential terror threat, the press service of the State Border Service said. "Due to the information about possible provocations on the common border and terrorism, our border guards temporarily increased control over citizens entering the country from Russia," the press service said, without giving further details. Local media said that restrictions would be applied to Russian men aged between 16 and 60 who travel without their families and have no invitation from the Ukrainian side. Previously, invitations or visas were not required for travel between Russia and Ukraine. The move came as pro-Russia activists attacked a military base and seized administrative buildings in several cities in eastern Ukraine, demanding a referendum on autonomy and closer ties with Russia. Ukrainian authorities suggested that Russian citizens were among the rebels, stirring the unrest. Moscow, however, has rejected the claims, saying that pro-Russian groups in Ukraine are comprised exclusively Ukrainian citizens.