The leaders of France, Germany, the US and UK have issued a statement on the nerve agent attack in the UK, saying there is "no plausible alternative explanation" than Russian culpability.
They condemned the "first offensive use of a nerve agent in Europe since the Second World War", calling it an assault on UK sovereignty.
The UK has expelled 23 Russian diplomats in the wake of the incident.
PM Theresa May on Thursday visited the site of the attack in Wiltshire.
"We do hold Russia culpable for this brazen, brazen act and despicable act," she said.
Russia has denied any involvement in the attack. It has vowed a swift response to the expulsion of its diplomats.
The nerve agent was used on a former Russian spy and his daughter in the city of Salisbury.
Sergei Skripal, 66, and Yulia Skripal, 33, remain in a critical condition in hospital.
What does the joint statement say?
The four nations say they "abhor the attack".
The statement says: "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."
The nations urged Russia to "address all questions related to the attack" and provide all details about the nerve agent used.
It added: "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."
Separately, the US has announced sanctions against Russian individuals and entities accused of trying to influence the 2016 election and carrying out cyber-attacks.
What does Russia say?
Russia says it had nothing to do with the attack and will respond to UK measures against it.
The Russian embassy said the decision to expel its diplomats was "unacceptable, unjustified and short-sighted".
The embassy has also suggested on its Twitter feed that the UK may have the capacity to manufacture the nerve agent used in Salisbury.
It said Russia closed all Soviet-era chemical weapons programmes in 1992 and some of the scientists involved were flown to other countries, including the UK.
"To identify a substance, formula and samples are needed - means UK has capacity to produce suspected nerve agent," it said.
What is the UK doing?
May on Thursday visited Salisbury. While there, she was briefed by public health experts, as Sergei and Yulia Skripal remain critically ill in hospital.
She also thanked emergency services and spoke to members of the public.
On Wednesday, the UK leader gave 23 Russian diplomats, identified as "undeclared intelligence officers", one week to leave, in the largest mass expulsion in the UK in more than 30 years.
Other measures taken against Russia include:
Increased checks on private flights, customs and freight
The freezing of Russian state assets where there is evidence they may be used to threaten life or property in the UK
The suspension of all planned high-level bilateral contacts
A World Cup boycott by ministers and the Royal Family.
The UK will also create a new chemical weapons "defence centre", Defence Secretary Gavin Williamson has said.
Announcing the measure, Williamson said Russia "should go away - it should shut up".
The facility is to be located at the Defence Science and Technology Laboratory in Porton Down, where scientists helped identify the nerve agent used against Skripal.