Sweden has expelled a Russian diplomat and Moscow has retaliated by throwing out a Swedish diplomat.
A Swedish Foreign Ministry spokesman said the Russian's activities had not been in accordance with the Vienna Convention, a diplomatic code of behavior.
But he did not disclose any details, nor did he say when either expulsion occurred.
Sweden has been highly critical of Russia's actions in Ukraine, and increased military activity by Russian planes and vessels in the Baltic Sea region has intensified regional tensions.
The Russian Foreign Ministry was not immediately available for comment.