Moscow (AFP) - A Russian opposition politician who fell into a coma due to poisoning this month has left the country for treatment abroad after his condition improved, his lawyer said Sunday.

"This morning Vladimir Kara-Murza flew out of the country... to go through rehabilitation treatment after his second acute poisoning," lawyer Vadim Prokhorov wrote on his Facebook page.

"The diagnosis in his hospital discharge report is still the same: 'toxic influence of an unknown substance'," he added.

Kara-Murza, 35, previously experienced sharp deterioration of health due to poisoning two years ago, which included kidney failure and nearly killed him.

Tests in laboratories abroad found high levels of heavy metals in his blood, but the Russian Investigative Committee denied his request to probe whether he was a target of intentional poisoning.

His family said that the latest collapse, which saw him put on a ventilator and renal dialysis in a Russian hospital, could be a result of the 2015 incident.

Kara-Murza was an ally of the late opposition politician Boris Nemtsov, who was shot dead close to the Kremlin in February 2015.

He currently works with the Open Russia foundation of Mikhail Khodorkovsky, a former oil tycoon who served 10 years in jail after openly opposing President Vladimir Putin.

Prokhorov said that Kara-Murza plans to continue his political work. The lawyer said that most likely Kara-Murza was intentionally poisoned.

Besides his work with Khodorkovsky, Kara-Murza has been deeply involved in lobbying in the United States for the expansion of the Magnitsky Act, which Prokhorov said "could be one of the reasons" for the poisoning because it targets Russian elites.

The 2012 sanctions list originally targeted Russian officials involved in the case of lawyer Sergei Magnitsky who died in prison after uncovering a government corruption scheme.

Since it was passed it has been expanded to include other individuals, including the Russian Investigative Committee chief Alexander Bastrykin, who was added only in January.