MOSCOW — The United States and two major European allies formally backed Britain’s claims of likely Russian links to a chemical toxin attack against a former spy, calling it the “first offensive use of a nerve agent” in Europe since World War II.

The joint statement from the leaders of France, Germany, the United States and Britain signaled another step in mounting international pressure on Russia over apparent ties to the assault.

The statement said the four nations shared the view of British investigators of Russian ties to last week’s attack against a former double agent and his daughter

There was no “plausible alternative explanation,” the statement added, noting that Russia’s “failure to address the legitimate request by the U.K. government further underlines its responsibility.”

“It is an assault on U.K. sovereignty and any such use by a state party is a clear violation of the Chemical Weapons Convention and a breach of international law,” said the statement, released by the office of the British prime minister.

“It threatens the security of us all,” it added, without spelling out any possible further reprisals by Britain and its allies.

The next move in the deepening standoff could come from Moscow.

Russia promised Thursday to respond “very soon” to Britain’s decision to expel 23 Russian diplomats, Foreign Minister Sergei Lavrov said. The British move was taken in response to the use of an alleged Russian nerve agent against a former Russian spy on British soil last week.

“The answer will come very soon, I assure you,” Lavrov said. “You know that we as polite people will first communicate this response to our British colleagues.”

Russia has been relatively slow to react to British Prime Minister Theresa May’s announcement on Wednesday that Britain would take action against Russia after Moscow ignored an ultimatum to explain how an alleged Russian nerve agent came to be used in Britain.

Moscow responded to the ultimatum with scorn and sarcasm, ultimately blowing off May’s demands. Meanwhile, officials and pundits in Moscow have issued a steady stream of denials and counterclaims, a tactic that has continued through Thursday.

Kremlin spokesman Dmitry Peskov said that President Vladimir Putin had met with members of his national security council Thursday for a “detailed discussion” on the situation with Britain. “Extreme concern was expressed in connection with the destructive and provocative position taken by the British side,” he said.

Lavrov reiterated earlier comments that the allegations were “boorish and unfounded.” The actions taken by the British “go beyond the limits of elementary rules of decency,” he said, while asserting that Russia has attempted to handle the situation in a civilized manner.

When asked how Britain might respond to any retaliation, Britain’s Defense Secretary Gavin Williamson said that Russia “should go away, it should shut up.”

He was taking questions after a speech announcing a $67 million investment into a new chemical weapons defense center.

In Brussels, NATO Secretary General Jens Stoltenberg condemned what he said was a “reckless pattern of Russian behavior over many years.” He added the alleged chemical weapons attack to the Kremlin’s ongoing nuclear buildup, military action in Georgia and Ukraine, and the targeting of Western political systems for influence operations.

“We do not want a new Cold War, and we do not want to be dragged into an arms race. An arms race has no winners. It is expensive, it is risky, it is in nobody’s interest,” Stoltenberg said. He said that any response to the chemical attack ought to be “proportionate, measured and defensive.”

Russia has also asked for access to the poison and its victims, 66-year-old Sergei Skripal and his daughter, 33-year-old Yulia Skripal.

They are both reported to be in comas after being found slumped on a park bench in the quiet town of Salisbury, near Stonehenge, on March 4. Skripal, a former Russian double agent, was jailed in Russia in 2006 for selling state secrets to British intelligence for 10 years, but he was released in 2010 as part of a high-profile spy swap.

Despite Russia’s constant and rigorous denials, the United States and France have fallen in behind Britain in support its conclusion that Russia was involved the use of the nerve agent on the Skripals.

“France agrees with the U.K. that there is no other plausible explanation,” President Emmanuel Macron’s office said in a statement following a phone call between Macron and the May.

Macron himself said France would take measures of its own in coming days against Russia.

British Foreign Secretary Boris Johnson has called on Britain’s allies to take a united stand against what he described as the “first offensive use of a nerve agent in Europe since World War II.”

Writing in The Washington Post, he said that “all responsible nations share an obligation to take a principled stance against this behavior,” which he characterized as part of a larger pattern of “reckless behavior” by Putin. The nerve agent, Novichok, was selected for a reason, he said.

“In its blatant Russian-ness, the nerve agent sends a signal to all who may be thinking of dissent in the intensifying repression of Putin’s Russia,” Johnson said. “The message is clear: We will find you, we will catch you, we will kill you — and though we will deny it with lip-curling scorn, the world will know beyond doubt that Russia did it.”

Analysts said that Britain was bracing itself for a tit-for-tat response from Russia.

“They are not going to take this lying down and we should expect that. If you’re not prepared to take a few blows, you shouldn’t make any punches. The question is, where does it stop?” said James Nixey, head of the Russia and Eurasia Program at Chatham House, a London-based think tank.

Adam reported from London. James McAuley contributed from Paris.