The EU's foreign policy chief called on Moscow Thursday to cooperate on an international probe into the poisoning of Russian opposition leader Alexei Navalny and said it would not rule out sanctions.

"The European Union calls upon the Russian Federation to fully cooperate with the Organisation for the Prohibition of Chemical Weapons (OPCW) to ensure an impartial international investigation," the bloc's top diplomat Josep Borrell said in a statement.

Brussels "reserves the right to take appropriate actions, including through restrictive measures," Borrell said.

The EU condemned "in the strongest possible terms the assassination attempt", the statement said.

Navalny, one of President Vladimir Putin's fiercest critics, fell ill on a flight last month and was treated in a Siberian hospital before being evacuated to Berlin.

Germany's claim that he was exposed to Novichok -- the same substance used against Russian ex-double agent Sergei Skripal and his daughter in the English town of Salisbury two years ago -- prompted widespread condemnation and demands for an investigation.

The EU said the use of chemical weapons "is completely unacceptable under any circumstances (and) constitutes a serious breach of international law and international human rights standards."

The Russian government "must do its utmost to investigate this crime thoroughly in full transparency and bring those responsible to justice. Impunity must not and will not be tolerated," Borrell said.

Wishing Navalny "a prompt and full recovery," the diplomat said that the EU would "continue to closely follow the issue and consider its implications."