Germany said Thursday its release of a convicted murderer in a prisoner swap with Russia was "not an easy decision" and called on Russia and Belarus to free all political detainees.

Russian citizen Vadim Krasikov was jailed for assassinating a former Chechen rebel commander in Berlin in 2019.

German government spokesman Steffen Hebestreit said the decision to send him back to Russia had been informed by Berlin's duty towards German citizens and solidarity with the United States.

"This was not an easy decision," he said in a statement.

He acknowledged there was a "public interest in enforcing a prison sentence on a convicted criminal".

But this had to be set against the "freedom, welfare and in some cases the lives of innocent people detained in Russia", he said.

"We hope that all those freed today will recover from their physical and mental ordeals with the support of their friends and family," said Hebestreit.

One of those freed was a German citizen who had been on death row in Belarus, he added.

"Our thoughts are with all those who are still imprisoned in Russia for expressing their opinions and for telling the truth about Putin's war of aggression in Ukraine," Hebestreit said.

He called on "the Russian and Belarussian leadership to release all unjustly imprisoned political detainees".