Vladimir Kara-Murza Jr. has been poisoned for a second time

A Russian opposition activist who survived being poisoned two years ago has fallen into a coma after he was poisoned for a second time.

Vladimir Kara-Murza Jr, 35, a journalist and a close associate of the murdered opposition leader Boris Nemtsov, was taken to a hospital last week with a sudden illness reminiscent of a mysterious poisoning he suffered two years earlier when he nearly died from kidney failure.

No cause for his kidney failure was determined, but in light of the fatal poisoning of defector Alexander Litvinenko and the mysterious deaths of other Russian opposition figures, many believed that Kara-Murza could have been deliberately poisoned.

The Russian activist's wife said on Tuesday that her husband had been poisoned again after he was rushed to hospital after being struck down with a sudden illness.

He has been put into a medically-induced coma and is currently fighting for his life in a Moscow hospital, where he is on life support.

Kara-Murza, an outspoken critic of Russian president Vladimir Putin, lives in a suburb of Washington with his wife and children, and was about to travel back home to the US when he was suddenly taken ill.

His lawyer Vadim Prokhorov said in a Facebook post later on Tuesday that the police had confirmed the diagnosis as poisoning by an unidentified substance.

Vladimir Kara-Murza Jr is a journalist and was a close associate of the murdered opposition leader Boris Nemtsov

Kara-Murza's wife, Yevgenia, told The Associated Press doctors have told her that her husband, who has been in a medically induced coma for several days, has now been diagnosed with an 'acute poisoning by an unidentified substance.'

He has been on a ventilator and undergoing renal dialysis since he collapsed in Moscow on Thursday.

'His condition is critical but stable,' said his wife Yevgeniya. 'The official diagnosis is acute poisoning by an unidentified substance.'

She said the family sent blood, hair, and fingernail samples to a private laboratory in Israel to determine the toxin. She would not speculate on the nature of the poisoning before those results are available.

His Washington-based wife also warned US president Donald Trump not to consider Vladimir Putin to be a friend.

'[Trump] must know that such people as Vladimir Putin are not friends. And they cannot be dealt with on friendly terms,' she said in an interview with ABC News.

Trump has faced criticism for refusing to condemn Putin as killer.

Vladimir Kara-Murza Jr was a prominent ally of murdered Nemtsov, a critic of Russian president Vladimir Putin

On Sunday, Fox News Host Bill O'Reilly asked the US president if he respects his Russian counterpart, to which to which Trump replied saying he did but added 'that doesn't mean I'm going to get along with him.'

When O'Reilly responded by saying: 'Putin's a killer,' Trump replied with: 'Lot of killers. We've got a lot of killers. What, you think our country's so innocent? You think our country's so innocent?'

The Kremlin said on Monday that it wanted an apology from Fox News over what it called the 'unacceptable' comments.

'We consider such words from the Fox TV company to be unacceptable and insulting, and honestly speaking, we would prefer to get an apology from such a respected TV company,' Kremlin spokesman Dmitry Peskov told reporters on a conference call.

Kara-Murza was previously taken ill in May 2015 at the offices of a state-owned legal news agency and rushed to hospital.

At the time his condition was said to be critical, but he later recovered from his injuries.

Family friend and opposition activist Andrei Bystrov said at the time: 'Doctors have just confirmed that he was poisoned.

As to what with, they can't say yet. It could be anything.'

Speaking after his recovery, Kara-Murza said that it was difficult to 'believe this was an accident', suspecting he had been intentionally poisoning, but said there was no way to be certain.

The Kremlin and its security forces deny involvement in a long line of political killings, from the high-profile poisoning of former KGB officer Alexander Litvinenko in London in 2006 to the assassination of Mr Nemtsov in February 2015.

Mr Kara-Murza worked on the release of an opposition report into the presence of Russian troops in east Ukraine.

Nemtsov was killed in February 2015 by drive-by shooter in the Russian capital Moscow

The report was begun by Mr Nemtsov before his murder, and alleged Russia had lost at least 220 soldiers since the start of fighting in east Ukraine in April 2014.

Mr Kara-Murza was also involved in the production of a documentary film about Chechnya's strongman leader, Ramzan Kadyrov.

The 30-minute documentary alleged the involvement of Mr Kadyrov, a former rebel fighter who went over to the Kremlin in 2000, in widespread murders and torture in the volatile North Caucasus republic.

The documentary was produced by Open Russia, a pressure group funded by Mikhail Khordokovsky, a former Russian oligarch and bitter critic of Russian President Vladimir Putin.