The leaders of the United States, France and Germany joined Britain on Thursday in blaming Russia for poisoning a former spy with a powerful nerve agent, condemning what they called the first attack with a nerve agent in Europe since World War II.

In a rare joint statement, US President Donald Trump, French President Emmanuel Macron, German Chancellor Angela Merkel and British Prime Minister Theresa May said “there is no plausible alternative explanation” to Russian responsibility.

They said Russia's failure to respond to Britain's “legitimate request” for an explanation “further underlines its responsibility” in the attack in southern England.

“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,” the leaders said, calling it “an assault on UK sovereignty” and “a breach of international law.”

Trump, who has often been reluctant to criticise Russian President Vladimir Putin, said it “certainly looks like the Russians were behind it.” “It's something that should never ever happen,” Trump said at the White House. “We're taking it very seriously as I think are many others.”

Trump spoke to reporters after his administration announced new sanctions on Russian entities for meddling in the 2016 US presidential election.

The four-nation statement is the fruit of British efforts to enlist international support as it tries to hold Russia accountable for the March 4 attack that left former Russian double agent Sergei Skripal and his daughter Yulia in critical condition and a British police officer seriously ill.

UK-Russia relations have plunged to Cold War-era levels of iciness since the poisoning. May on Wednesday expelled 23 Russian diplomats, severed high-level contacts with Moscow and vowed both open and covert actions following the attack.

Russia denies being the source of the nerve agent that poisoned the Skripals. Kremlin spokesman Dmitry Peskov said Russia was “worried by this situation” and would work to express its position on the international stage.

Russian Foreign Minister Sergey Lavrov said on Thursday that Moscow would “certainly” expel some British diplomats in a tit-for-tat response.

In remarks carried by the RIA Novosti news agency, Lavrov said the move would come “soon.”

Lavrov said British accusations of Moscow's involvement are intended to distract public attention from the UK's troubled exit from the European Union.

He argued that “boorish and unfounded” accusations against Russia “reflect the hopeless situation the British government has found itself in when it can't meet the obligations given to the public in connection with the exit from the EU.”

Kremlin spokesman Peskov said the decision about how to retaliate would come from Putin, “and there is no doubt that he will choose the option that best reflects Russian interests.”

UK Defense Secretary Gavin Williamson called relations between the two countries “exceptionally chilly” and said Russia should “go away and shut up.”

And British Foreign Secretary Boris Johnson said Russia targeted Skripal a former Russian intelligence officer convicted of spying for Britain to make it clear that those who defy the Russian state deserve to “choke on their own 30 pieces of silver.”

“The reason they've chosen this nerve agent is to show that it's Russia, and to show people in their agencies who might think of defecting or of supporting another way of life, of believing in an alternative set of values, that Russia will take revenge,” Johnson told the BBC.

May announced the sanctions against Russia in the House of Commons after Moscow ignored a deadline to explain its links to the attack on the Skripals.

She said Britain had cancelled an invitation for Lavrov to visit and would not send ministers or royals to the soccer World Cup this summer in Russia though the England team will still compete in the tournament.

May said Britain would also clamp down on murky Russian money and strengthen the government's ability to impose sanctions on those who abuse human rights, though she gave few details.

Britain is also trying to build a unified Western response, saying the attack in Salisbury is just the latest example of Russia's disregard for international norms on the rule of law.

In their joint statement, Trump, Macron, Merkel, and May cited “a pattern of earlier irresponsible Russian behavior” and called on Russia to disclose details of its Novichok program to the Organisation for the Prohibition of Chemical Weapons.

“We call on Russia to live up to its responsibilities as a member of the UN Security Council to uphold international peace and security,” they said.

The leaders did not say what, if any, actions they would take if Russia does not comply.

On Thursday, May visited Salisbury, meeting police and medics and viewing the site where the Skripals were found critically ill.