France, Germany, Britain and US have released a joint statement blaming Russia for the poisoning of an ex-spy in England and saying the case 'threatens the security of us all'.
The quartet said the attack on Sergei Skripal and his daughter was "the first offensive use of a nerve agent in Europe since World War II".
Moscow has denied any responsibility for the attack.
"We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury, UK, on 4 March 2018," the statement said.
"This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War.
"It is an assault on UK sovereignty and any such use by a State party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all."
It comes after Russia had earlier reacted angrily to Theresa May's decision to expel 23 diplomats from the UK.
In an interview with Russian news agency Sputnik, Foreign Minister Sergei Lavrov said Russia would definitely eject British diplomats in retaliation.
Earlier, a foreign ministry spokeswoman had described the allegations of Russian involvement in the poisoning of Sergei Skripal as "insane", and accused the UK of failing to cooperate with Moscow in investigating the incident.
"We plan to introduce reciprocal steps. They are currently being worked on and will be adopted in the near future," Maria Zakharova said.
Experts have claimed that novichok, the nerve agent used on Skripal, has only ever been produced at a single site in Russia.
However Russia told the UN security council that it has never made novichok.