An exiled Russian Opposition MP has told the ABC he has "no doubt" the suspected poisoning of one of the country's most prominent Opposition activists was authorised by senior members of the Putin regime.

Key points: Vladimir Kara-Murza on life support after being poisoned with unidentified substance

Vladimir Kara-Murza on life support after being poisoned with unidentified substance Activist was instrumental in convincing US to impose sanctions on Russian officials

Activist was instrumental in convincing US to impose sanctions on Russian officials Many journalists, activists and lawyers have met 'messy ends' in Russia: Expert

Opposition activist and writer Vladimir Kara-Murza has been in a coma in a Moscow hospital since he fell ill last Thursday as he prepared to travel to his US home for his daughter's birthday.

His wife Evgenia Kara-Murza said doctors have told her he has been poisoned by an unidentified substance.

"His condition is critical but stable," she said.

"The official diagnosis is acute poisoning by an unidentified substance.

"He is still on life support for all his major organs."

Mr Kara-Murza, 35, survived a similar suspected poisoning in 2015.

He was instrumental in convincing the US to impose sanctions on Russian officials in 2012 for the death in custody of Russian lawyer Sergei Magnitsky.

Russian opposition MP Ilya Ponomarev, who lives in exile in Ukraine and is an ally of Mr Kara-Murza, has told the ABC's PM program he believes his friend has been punished for his role in the so-called Magnitsky Act.

"For me, there is no doubt that authorisation for such a thing and for the [attempted] assassination of such a public figure like Vladimir could have only come from higher circles of the current authorities," Mr Ponomarev told PM.

"I am not saying it was ordered directly by [Russian President] Vladimir Putin, but it has to be ordered by somebody very close to him."

"Somebody very close" to Vladimir Putin must have ordered the attack, Ilya Ponomarev says. ( Reuters: Maxim Zmeyev )

'Unknown poisons generally produced in state labs'

Russia expert Matthew Sussex, from the Australian National University, is not surprised Mr Kara-Murza's family and allies are pointing the finger at the Russian Government.

"It seems that Vladimir Kara-Murza has been poisoned with something that doctors have absolutely no idea what it is but is very similar to something he was previously poisoned with in 2015," Associate Professor Sussex said.

"Unknown toxins are things which are generally only produced in state laboratories.

"There's been a fairly extensive history of journalists, activists, lawyers who have met messy ends in Russia, either through being shot, stabbed or having heart attacks and, in this case particularly, being poisoned."

Mr Kara-Murza's close ally, Opposition politician Boris Nemtsov, was assassinated near the Kremlin in 2015.

In 2006, former Russian spy Alexander Litvinenko died after being poisoned by a rare radioactive material in London, a murder which a British inquiry found was "probably approved" by Mr Putin.

Earlier this week, US President Donald Trump said he respected Mr Putin, in an interview on American TV.

When the host interjected that "Putin's a killer," Mr Trump said, "We've got a lot of killers. What, you think our country's so innocent?"

Ms Kara-Murza has joined US Democrats and some Republicans in their condemnation of Mr Trump.

"He has to understand that this is not the way to lead a country," she told ABC America.

"He must know that such people as Putin are not friends and that they cannot be dealt with on friendly terms.

"The United States have to stand up for their principles."