Russia accused a Ukrainian diplomat on Saturday of trying to obtain classified information and ordered him to leave the country by April 22, prompting a like-for-like response from Ukraine as border tensions simmer.
Russia's FSB security service said Oleksandr Sosoniuk had been detained when he tried to access information from Russian law enforcement databases during a meeting with a Russian citizen.
The foreign ministry gave Sosoniuk 72 hours to leave, and Ukraine then did likewise to a Russian diplomat in Kyiv.
Ukraine's foreign ministry said Sosoniuk had been held for several hours before being allowed to return to his consulate in St Petersburg, and called the detention a provocation.
"We fully deny the accusations levelled against the consular officer," it said in a statement.
Tensions between Moscow and Kyiv have been rising amid a build-up of Russian troops along the border and clashes in eastern Ukraine between the army and pro-Russian separatists.
Ukrainian President Volodymyr Zelenskiy on Friday pressed for peace talks with Moscow.
(Reporting by Polina Devitt in Moscow and Natalia Zinets in Kyiv; Editing by Kevin Liffey)
