The leaders of Britain, France, Germany and the United States said there was "no plausible alternative explanation" for the use of the Soviet-designed chemical Novichok in the English city of Salisbury.
In a joint statement, they demanded Moscow "address all questions" related to the attack against former Russian spy Sergei Skripal, which they said amounted to a "breach of international law".
Skripal moved to Britain in a 2010 spy swap and had taken his daughter Yulia, who was on a visit, out for lunch before they both collapsed on a bench in the street on March 4.
The investigation into the suspected nerve agent attack on Russian double agent Sergei Skripal and his daughter Yulia continues. Source: AAP
In her first visit to Salisbury, where she also met a police officer injured in the incident, British Prime Minister Theresa May said it was important to send a "united" response.
"This happened in the UK but it could have happened anywhere and we take a united stance against it," she said.
US President Donald Trump said: "It certainly looks like the Russians were behind it. Something that should never, ever happen, and we're taking it very seriously."
Russia response 'soon'
May on Wednesday announced the expulsion of 23 Russian diplomats and suspended high-level contacts, including announcing that British royals and ministers would boycott this summer's football World Cup in Russia.
She warned more measures could follow, noting that the US-led NATO alliance and the UN Security Council had discussed the attack, while it was also expected to be on the agenda of the European Union summit next week.
The crisis comes as Russia prepares for a presidential election on Sunday in which Vladimir Putin is expected to easily win a fourth term.
Russia has repeatedly denied any involvement in the attack, the first offensive use of a nerve agent in Europe since World War II.
Foreign Minister Sergei Lavrov said Russia would respond by expelling British diplomats "soon".
He said his country had no motive to target Skripal, but suggested other players could use the poisoning to "complicate" the World Cup.
He also accused Britain of trying to "deflect attention" from its decision to leave the European Union, which has cast uncertainty over its place in the world.
Pattern of behaviour
NATO allies on Wednesday put out a joint statement condemning the attack, and Secretary General Jens Stoltenberg said it must have "consequences".
He said Britain had not invoked the alliance's Article 5 mutual defence clause.
But he noted the attack came against the "backdrop of a pattern of reckless behaviour" by Russia - a point also made in the US, British, German and French statement.
"The Soviet Union or Russia had no programmes to develop a toxic agent called Novichok": Sergei Ryabkov. Source: Getty Images
Separately, Washington on Thursday levied sanctions against Russia's top spy agencies and more than a dozen individuals for trying to influence the 2016 US presidential election and two separate cyberattacks.
The measures target five entities and 19 individuals - including the FSB, Russia's top spy service; the military intelligence agency, or GRU; and 13 people recently indicted by Robert Mueller, the US special counsel handling a sprawling Russia probe.
Sanctions were also levied against individuals behind the separate Petya cyberattack and an "ongoing" attempt to hack the US energy grid.
'Full disclosure'
The British, US, French and German leaders called on Russia to provide "full and complete disclosure" of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons (OPCW).
But deputy foreign minister Sergei Ryabkov said Moscow never had any such programme.
"I want to state with all possible certainty that the Soviet Union or Russia had no programmes to develop a toxic agent called Novichok," he told Interfax news agency.
He slammed people "distributing information that the programme allegedly existed," an apparent reference to Soviet chemist Vil Mirzayanov, who first revealed the existence of that class of ultra-powerful nerve agents.
Russia says it has destroyed all its chemical weapons and shut down its military chemical programme in the mid-1980s.
Britain had demanded Russia disclose details of the programme to the OPCW by midnight on Tuesday, but Moscow refused to respond to the "language of ultimatums".
Foreign ministry spokeswoman Maria Zakharova said May's statements were "completely crazy accusations", and accused Britain of failing to work with Moscow through the OPCW.
Russia has complained that it has had no access to nerve agent to be able to analyse it.
However British Foreign Secretary Boris Johnson told the BBC that Britain is "entirely in conformity" with OPCW procedures and is sending a sample of the nerve agent to the watchdog for examination.
"Frankly, Russia should go away and shut up," British Defence Secretary Gavin Williamson said.
