Ukraine said on Thursday it will impose stricter border controls on Russian men trying to enter the country, where pro-Moscow rebellions have broken out.
"From today the Ukrainian border control service has significantly increased checks at the border with Russia, not only at the airport," a spokesman for the service, Oleh Slobodian, said.
"This applies to Russian citizens because there is information about possible provocations at the border, up to and including terrorist attacks. Attention will be primarily paid to men of an active age, traveling alone or in a group."
Earlier on Thursday, Russian airline Aeroflot issued a statement warning passengers that the Ukrainian authorities were "massively restricting" entry to the country.
It indicated men between the ages of 16 and 60 would be denied entry unless they had proof of a reason to visit, such as family ties or an invitation from a business or individual, or were travelling with their families.
Most Ukrainian passport holders who are residents of Crimea - which Russia annexed last month - will also face stricter checks at the border, the airline said.