German Chancellor Angela Merkel paid a personal visit to Russian opposition politician Alexei Navalny while he was undergoing treatment in a Berlin hospital for poisoning, in what Vice Chancellor Olaf Scholz on Monday called a “murderous attack”.

Scholz’s comment and the news of Merkel’s visit are likely to annoy Moscow, which rejects the finding of German, French and Swedish experts that Navalny was poisoned with a nerve agent in Russia last month. Russia has repeatedly criticised Germany over what it says is a failure to share information on the case.

“It was a personal visit to Navalny in hospital,” Merkel’s spokesman Steffen Seibert told a regular news conference, declining to disclose details of what was said or how long the meeting lasted.

Navalny wrote on Twitter that it was “a private meeting and conversation with the family”. He added: “I am very grateful to Chancellor Merkel for visiting me in hospital.”

Navalny was flown from Russia to Berlin last month after falling ill on a domestic flight. He received treatment in the Charite hospital for 32 days before being discharged last week.

Scholz said international organisations had independently confirmed that Navalny was poisoned with the Soviet-style nerve agent Novichok, and he urged Moscow to cooperate in the investigation into what happened.

“It was a brutal, murderous attack,” Scholz told members of the VAP foreign press association in Berlin.

“And that is why we call on Russia to cooperate in the investigation. And of course, Europe will have to decide together on an appropriate response,” Scholz added.

Germany holds the rotating presidency of the 27-nation EU.

The Navalny case has further worsened relations between Moscow and a number of Western countries. Merkel has faced calls to halt the nearly-completed Nord Stream 2 pipeline, which is meant to bring more Russian gas directly to Germany.

Asked if European sanctions against Russia should include Nord Stream 2, Scholz said the pipeline was a privately owned project of nearly 100 companies, adding that its completion would reduce Germany’s dependence in the energy sector.

Nord Stream 2 is led by Russia’s state gas giant Gazprom, with half of the funding provided by Germany’s Uniper and BASF’s Wintershall unit, Anglo-Dutch company Shell, Austria’s OMV and Engie.