The leaders of France, Germany, the US and the UK have issued a rare joint statement, saying they 'abhor' the nerve agent attack in Salisbury.
They have also said Russia is likely responsible, claiming "there is no plausible alternative explanation".
Former Russian spy Sergei Skripal and his daughter Yulia were targeted in the attack, and remain in a critical condition in hospital.
They were found unconscious on a bench in Salisbury almost two weeks ago.
A British policeman also remains seriously ill.
Mr Skripal was convicted in Russia in 2006 of passing state secrets to Britain, before later being given refuge in the UK as part of a spy swap.
British Prime Minister Theresa May has said it was 'highly likely' Russia was behind the attack - a claim Russia has denied.
Mrs May today issued a fresh statement, alongside her counterparts Angela Merkel, Emmanuel Macron and Donald Trump.
The other leaders say they share the assessment of the UK that Moscow was likely responsible - noting they have been "thoroughly briefed" by British officials on the matter.
They argue that Russia's failure to respond to a UK demand for an explanation "further underlines its responsibility".
The leaders say: "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."
They add: "We call on Russia to address all questions related to the attack in Salisbury.
"Our concerns are also heightened against the background of a pattern of earlier irresponsible Russian behaviour. We call on Russia to live up to its responsibilities as a member of the UN Security Council to uphold international peace and security."
'Hype and propaganda'
Meanwhile, the Russian ambassador to Ireland has described the British response to the Salisbury attack as ‘hype’ and ‘propaganda'.
Moscow is expected to expel a number of British diplomats in response to the expulsion of 23 diplomats from the UK.
Claims of Russian responsibility were today rejected by the Russian ambassador to Ireland, Yury Filotov.
The Russian Ambassador to Ireland describes reaction to #SalisburyChemicalAttack as ‘hype & propaganda’ pic.twitter.com/XFMArzMpvz
— Paul Quinn (@pdquinn7) March 15, 2018
Ambassador Filotov argued: "Instead of going through normal, legal and logical ways, British authorities chose to really hype... [a] campaign against Russia."
He suggested the British response was "unprecedented in its provocative manner".
