Following news of the suspected poisoning of Russian opposition leader Alexei Navalny, White House national security adviser, Robert O’Brien, said the news was “extraordinarily concerning” and could affect US-Russia relations.

“He’s a very courageous man. He is a very courageous politician to have stood up to Putin inside Russia, and our thoughts and our prayers are with him and his family,” O’Brien told Fox News. “If the Russians were behind this ... it’s something that we’re going to factor into how we deal with the Russians going forward.”

The Telegraph reports that German chancellor Angela Merkel has also expressed concern at the incident, as has French president Emmanuel Macron. Both of the heads of state offered to treat him in their countries.

“I hope that he can recover and naturally whether it be in France or in Germany he can receive from us all the help and medical support needed,” Merkel said in a joint news conference with Macron.

Navalny is currently in a coma and on a ventilator in the Russian city of Omsk, where, it has been reported, doctors are refusing to allow him to leave the country for treatment abroad, despite reservations expressed by his family regarding the reliability of any Russian hospital in his treatment.