The leaders of France, Germany, the United States, and Britain say Russian responsibility is the "only plausible explanation" for the poisoning of a former spy with a military-grade nerve agent in England 11 days ago.
A joint statement on March 15 said the leaders “abhor” the attack on Sergei Skripal and his daughter, calling it an assault on British sovereignty and "a breach of international law."
Skripal and his daughter Yulia remain critically ill after being found unconscious on a bench in the city of Salisbury on March 4.
Britain says the chemical used in the attempted murder was identified as part of a group of nerve agents developed by the Soviet military known as Novichok.
"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 French, German, U.S., and British leaders said.
The joint statement came a day after British Prime Minister Theresa May announced the expulsion of 23 Russian diplomats, who she said were spies, and other measures in retaliation for what Britain is treating as attempted murder.
Separately, U.S. President Donald Trump said "it looks like" Russia was behind the poisoning.
"It looks like the Russians were behind it," Trump said during a meeting at the White House with Ireland's prime minister, Leo Varadkar, on March 15.
Russia denies involvement in the incident, and Foreign Minister Sergei Lavrov vowed a swift response to what President Vladimir Putin's spokesman called Britain's "destructive" moves.
"Great concern was expressed about the destructive and provocative stance taken by Britain," Kremlin spokesman Dmitry Peskov said after Putin and senior officials discussed the issue at a meeting of the Russian presidential Security Council.
Lavrov said that a Russian "response will come very soon, I assure you."
In their statement, the Western leaders called on Russia to provide full and complete disclosure of its Novichok nerve agent program to the Organization for the Prohibition of Chemical Weapons (OPCW), the international body set up to stop chemical warfare.
French President Emmanuel Macron told journalists he will "announce in the coming days the measures that we intend to take," saying "everything leads us to believe that responsibility is in fact attributable to Russia."
In Brussels, NATO Secretary-General Jens Stoltenberg said that Britain could count on NATO's solidarity, adding there has been no request by London to activate the alliance's mutual-defense clause.
Stoltenberg also said that the "unacceptable" attack in Salisbury was part of “a reckless pattern of Russian behavior over many years."
He cited Russia’s aggression in Ukraine, its military presence in Georgia and Moldova “against these countries’ will,” “meddling” in the Western Balkans, "attempts to subvert democratic elections and institutions," and the “military buildup from the North of Europe to the Middle East.”
Peskov said that Moscow considers Britain's stance "irresponsible" and repeated Russia's claim that Britain has provided no proof to back up its accusations against Russia.
He also said that the matter won't disrupt the March 18 presidential election that Putin -- who is popular in part because of his defiance of the West and who has stepped up control over Russian politics over nearly two decades in power -- seems certain to win.
Lavrov referred to the British allegations against Russia as "boorish," and said they "reflect the hopeless situation the British government has found itself” over the country's exit from the European Union.
Earlier, the Russian Foreign Ministry called Britain’s accusations “completely insane."
May on March 14 announced the biggest expulsion of Russian diplomats in 30 years, the suspension of planned high-level contacts with Russia, and a boycott by government officials and royal family members of the soccer World Cup that Russia is hosting in June and July.
“There is no alternative conclusion, other than that the Russian state was culpable for the attempted murder" of Skripal, 66, and his 33-year-old daughter, "and for threatening the lives of other British citizens” in Salisbury, May said.
Condemning the "unlawful use of force by the Russian state against the United Kingdom,” May said Britain "will freeze Russian state assets wherever we have the evidence that they may be used to threaten the life or property" of its citizens or residents.
In separate interview with BBC television and BBC radio, Foreign Secretary Boris Johnson on March 15 defended the measures as the British government is under pressure from lawmakers and media to show it was getting tough on Russia.
He said that the country’s law enforcement agencies were investigating rich Russian individuals with assets in Britain and suggested that those who owe their wealth to their ties with President Vladimir Putin could be brought to justice.
The foreign secretary also said that the attack was Russia's "way of saying to people this is what happens to people who stand up to our regime."
"Now is the moment for Putin to jam the lid down and send a message to people: 'You do this, you're going to die,’" he added.
Johnson also said a sample of the nerve agent would be sent to the OPCW for analysis.
Skripal, a retired Russian military intelligence colonel, was convicted by a Moscow military court in 2006 of high treason for passing secrets to Britain's Secret Intelligence Service, commonly known as MI6.
He was one of four Russian prisoners released in 2010 in exchange for 10 Russian sleeper agents detained in the United States. He later became a British citizen, British authorities said.