Germany said it had expelled two Russian embassy employees in protest over what it said was Moscow's lack of cooperation in an investigation into the murder of a Georgian man in Berlin, in which prosecutors suspect Russian or Chechen involvement.
Russia, which denies any involvement, responded swiftly, saying it would retaliate for what it called Germany's "unfriendly" move.
Berlin's decision marks an escalation in already heightened tensions between Russia and Germany and other Western countries following the poisoning last year of a former Russian spy, Sergei Skripal, and his daughter on British soil.
"With this step, the German government is reacting to (the fact that) Russian authorities have not cooperated sufficiently in the investigations surrounding the murder of Tornike K. in a park in Berlin," said the German foreign ministry on Wednesday.
The victim, also known as Zelimkhan Khangoshvili, was shot twice in the head in a central Berlin park in August as he was heading to a mosque. He had previously fought alongside anti-Moscow separatists in Russia's autonomous region of Chechnya.
Prosecutors said there were sufficient leads to indicate that the killing was ordered either by the Russian state or by Chechen authorities.
Russia's foreign ministry responded angrily.
"We view statements from the German side about the expulsion of two employees of the Russian Embassy in Berlin as unfounded and unfriendly. We'll be forced to take a series of reciprocal measures," Russian news agencies quoted the ministry as saying.
The Kremlin has previously denied any Russian government involvement in the murder.