Ukraine is stepping up its border controls with Russia, restricting entry for Russian men of "fighting age," a spokesman for the State Border Guard Service of Ukraine told RFE/RL.
"I can confirm that we temporarily have been forced to strengthen border control efforts in connection with information about possible provocations at the border, and even possible terrorist acts," said Oleh Slobodyan. "Of course we are looking first and foremost at men, you might say, of a fighting age that are traveling alone or in small groups."
Russia's Foreign Ministry has demanded an explanation from Ukraine. It said Russian authorities would consider "possible retaliatory measures."
In an apparently related development, the Russian Border Guard press service said April 17 that 45 Russian passengers on a train headed to Ukraine were stopped at the Ukrainian border and ordered by Ukrainian border guards to go back.
In its statement, Aeroflot said that only male Russian nationals arriving in Ukraine to see relatives, or care for sick relatives, or attend a funeral will be permitted to enter the country. It said such Russian male passengers must have documents proving the purpose of their visit.
Aeroflot said similar restrictions apply to residents of Crimea, the Ukrainian territory that was annexed by Russia in March.
Aeroflot said Crimean women between the ages of 20 and 35 will have to undergo special checks on arrival at Ukrainian airports.
Aeroflot recommended in its statement that Russian citizens avoid traveling to Ukraine at this time.
The action comes as separatist, pro-Russian militants have caused increasing unrest in Eastern Ukraine. Ukrainian security forces, in turn, have launched an operation against the separatists, which has led to 3 casualties among the pro-Russian activists.
Ukraine and the United States have accused Russia of fomenting the unrest, but Russian President Vladimir Putin called such claims "nonsense." Russia has amassed its forces near the border with Ukraine.