Russian dissident Vladimir Kara-Murza said his mother was in hospital in Berlin on Tuesday, while German police said they were investigating a case of attempted homicide.

Kara-Murza said suspicions that his mother, a German-Russian national, had been poisoned had not been confirmed.

Police said the results of toxicology tests were not expected before Wednesday.

"My mother is indeed in a hospital in Berlin, but suspicions of poisoning or heart attack have thankfully not been confirmed," Kara-Murza posted on social media. "The doctors are continuing with the evaluation."

Had complained of poisoning symptoms

Police said the woman, whose name they did not give, was being moved to the Charite hospital's isolation wing. She had gone to hospital in the afternoon complaining of poisoning symptoms.

"We are currently looking into whether there is any evidence of a political motivation," police spokesperson Jane Berndt said. She added that the victim's flat, in the western district of Charlottenburg-Wilmersdorf, was being searched for clues.

Kara-Murza, a prominent critic of Russian President Vladimir Putin and his full-scale invasion of Ukraine, was one of more than 20 prisoners that Russia, Belarus and the West swapped in August, in the largest prisoner exchange since the Cold War.

Among those for which he was swapped was Russian assassin Vadim Krasikov, convicted of the 2019 murder in a Berlin park of a Georgian citizen who had been part of the Chechen independence movement.

Last year, Russian investigative journalist Elena Kostyuchenko was taken ill on a train to Berlin after what she believed was a poison attack. Police opened an investigation into a case of attempted murder.