The European Union has threatened sanctions against Russia while NATO has called an urgent meeting to discuss Germany's finding that opposition leader Alexei Navalny was poisoned by a Soviet-era nerve agent.
Western leaders are demanding answers from Moscow after Berlin said Wednesday there was "unequivocal evidence" that the 44-year-old Kremlin critic had been afflicted by the infamous nerve agent Novichok.
The Kremlin furiously denies the claims.
The EU's foreign policy chief Josep Borrell called on Moscow to cooperate with an international probe into the poisoning and said the 27-nation bloc would not rule out sanctions.
Brussels "reserves the right to take appropriate actions, including through restrictive measures", Mr Borrell said in a statement.
Mr 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 European Union condemned "in the strongest possible terms the assassination attempt", the Borrell statement said.
The EU said the use of chemical weapon "is completely unacceptable under any circumstances (and) constitutes a serious breach of international law and international human rights standards."
