German Chancellor Angela Merkel on Wednesday said that Russian Opposition leader Alexei Navalny, who is in a serious condition at a Berlin hospital, was poisoned with a Soviet-style Novichok chemical nerve agent, Reuters reported.

“This is disturbing information about the attempted murder through poisoning against a leading Russian opposition figure,” Merkel said at a press conference. “Alexei Navalny was the victim of an attack with a chemical nerve agent of the Novichok group. There are very serious questions now that only the Russian government can answer, and must answer.”

Navalny, an outspoken critic of Russian President Vladimir Putin, collapsed on a flight from Siberia to Moscow on August 20 after drinking tea at the airport. His allies said he was deliberately poisoned, but Russian authorities have said there is no evidence of this. On August 22, he was brought to Berlin’s Charite hospital for medical treatment. Tests carried out by the Army in consultation with the hospital revealed “unequivocal evidence” of poisoning, the German government said.

Russia has asked Germany to provide test results for information of the medical tests, including drugs, poisons, heavy metals and cholinesterase inhibitors, which affect the nervous system, according to Reuters. Moscow has also denied involvement in the incident and the Russian foreign ministry said Germany’s assertion was not backed by evidence.

Merkel said Germany would consult its NATO allies about how to respond to the situation. German Foreign Minister Heiko Maas urged Russia to conduct an investigation. “We condemn this attack in the strongest terms,” he added.

Novichok, a potent class of chemical weapon developed by the Soviet Union, is the same substance used on former spy Sergei Skripal and his daughter in England in 2018. Russia has repeatedly maintained that it was not involved in the attack, which the Skripals survived. The Novichok revelation provoked outrage from other countries, who demanded an explanation from Moscow.

The White House said the use of Novichok was “completely reprehensible”, while the United States National Security Council said that Washington would work with allies to hold those in Russia accountable. The United Kingdom and Italy also called on Russia to explain the incident. British Prime Minister Boris Johnson called the use of a chemical weapon “outrageous”.

Meanwhile, Charite hospital said Navalny continued to be in a serious condition in an intensive care unit, but added that some of his symptoms were receding. Long-term consequences from his poisoning cannot be ruled out, the hospital said.