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, calling the attack “the first offensive use of a nerve agent in Europe since the Second World War.”

In a rare joint statement, U.S. 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.”

The leaders said the use of a chemical weapon is “an assault on U.K. sovereignty” and “a breach of international law.”

The 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.

U.K.-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. Dmitry Peskov, the Kremlin spokesman, said Russia was “worried by this situation” and would work to express its position on the international stage.

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

Lavrov, in remarks carried by the RIA Novosti news agency, said the move would come “soon,” but added that Moscow would inform London through official channels before publicly announcing its countermeasures.

Lavrov said British accusations of Moscow’s involvement are intended to distract public attention from the U.K’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 exit from the EU.”

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

U.K. 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.

As a result of the suspension of high-level contacts with Russia, Britain canceled an invitation for Lavrov to visit. British ministers and royals also won’t attend the World Cup soccer tournament this summer in Russia, May said.

May said that 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 Organization 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.

May on Thursday went to the British city of Salisbury, to view the site where the Skripals were found critically ill.

(AP)