The Ukrainian Foreign Ministry has ordered the expulsion of Russia's military attache, saying he was caught "red-handed" receiving classified information on Ukraine's cooperation with NATO.
The ministry said Thursday that the diplomat had been detained a day earlier and declared persona non grata on Ukrainian territory.
The Vienna Convention's persona non grata declaration allows countries to expel foreign diplomats who would otherwise be protected from prosecution by their diplomatic status.
The man was a Russian intelligence officer who had been collecting intelligence on "Ukrainian-NATO military and political cooperation," said Ukrainian Security Services spokeswoman Maryna Ostapenko.
"On April 30, he was caught red-handed receiving classified material from his source," Ostapenko said, adding that the source as a colonel in the Ukrainian armed forces.
Ukraine has accused Russia of supporting the recent occupation of several administrative and government buildings in Ukraine's industrial east by pro-Russian separatists.
Russia denies having any part in the rebellion, but has warned it reserves the right to intervene to protect ethnic Russians and has massed tens of thousands of troops on its western frontier with Ukraine.
Ostapenko said the attaché had been handed over to the Russian Embassy and ordered to leave, though she was not sure if he had yet done so.
There was no immediate response from Moscow, which was observing the May 1 public holiday.