BERLIN • Chancellor Angela Merkel demanded full transparency from Russian President Vladimir Putin's government after a German medical team determined that Mr Alexei Navalny, the Russian leader's most prominent critic, was likely poisoned last week.

"In view of Navalny's major role in Russia's political opposition, the country's authorities are urgently called upon to fully investigate this act as a matter of urgency - and to do so in a completely transparent way," Dr Merkel said in a joint statement with her foreign minister Heiko Maas. "Those responsible must be identified and brought to justice."

The demand marks a sharp escalation in tensions between the West, led in this case by Dr Merkel's government, and the Kremlin. A United States official warned earlier on Monday that confirmation he was poisoned would turn the case into a major issue.

The European Union's diplomatic chief Josep Borrell has called on the Russian authorities to launch an "independent and transparent investigation" into the apparent poisoning.

"The European Union strongly condemns what seems to be an attempt on Mr Navalny's life," said Mr Borrell on Monday.

Two days after Mr Navalny was transferred to the German capital for medical treatment, Berlin's Charite hospital said that the anti-corruption activist is in serious condition but there is no acute threat to his life.

However, he may suffer long-term damage to his nervous system, according to the statement. Mr Navalny has been in a medically induced coma since last Thursday after falling ill, and was evacuated from Russia on Saturday.

His allies blamed Russia's security services for the poisoning.

But Russia yesterday brushed off the call for an investigation. "If the substance is identified and it's shown to be a poisoning, that would be basis for an investigation," Kremlin spokesman Dmitry Peskov said on a conference call. For the moment, he said, it's up to the German clinic to determine what Mr Navalny was exposed to.

In Berlin, doctors found evidence of poisoning through a substance related to cholinesterase inhibitors, said Charite.

The specific substance was not immediately known and will require further testing to be identified, the hospital said, adding that the patient is being treated with the antidote atropine and his prognosis remains unclear.

BLOOMBERG, AGENCE FRANCE-PRESSE