MOSCOW — Russia vowed Tuesday to retaliate if Britain imposes sanctions in response to a chemical attack on British soil and demanded access to samples of a nerve agent that British investigators say they have identified as Russian.

Foreign Minister Sergei Lavrov also said Russia does not intend to comply with British Prime Minister Theresa May’s demand Monday for an official explanation of how a nerve agent identified as Novichok, which was developed by the former Soviet Union, allegedly came to be used in the poisoning attack in southern England that targeted a former Russian spy and his adult daughter.

Lavrov insisted that Russian experts should be able to examine the British evidence but again denied Russian involvement in last week’s attack.

There has, however, been a history of Russian emigres dying in murky and sometimes not-so-murky circumstances in Britain.

Russian's foreign minister confirmed March 16 the country would expel British diplomats after Britain made a similar move for the poisoning of Sergei Skripal. (The Washington Post)

And in the midst of the controversy over the chemical attack, British counterterrorism police said Tuesday they are investigating the unexplained death of another Russian national, Nikolai Glushkov, 68, in London. His body was found Monday, and police said they see no connection between the cases.

Glushkov had been an associate of the oligarch Boris Berezovsky, who had fallen afoul of Russian President Vladi­mir Putin and lived in exile in Britain until his death by strangulation in 2013.

May spoke Tuesday afternoon with President Trump about the assault on the former spy, Sergei Skripal. She told him it was “highly likely that Russia was responsible for the attack,” according to a statement released by the British Embassy.

“President Trump stated the United States stands in solidarity with its closest ally and is ready to provide any assistance the United Kingdom requests for its investigation,” the White House stated. “President Trump agreed with Prime Minister May that the government of the Russian Federation must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom.”

Earlier, though, the president had hedged on the issue of blame.

“We’re speaking with Theresa May today, and as soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be,” Trump said when asked about it by reporters outside the White House. “It sounds to me like it would be Russia based on all of the evidence they have.”

Secretary of State Rex Tillerson, speaking en route to Washington from Africa on Monday evening, said the nerve agent “clearly came from Russia,” and he warned of consequences. Hours after Tillerson backed the British accusation, the White House announced Tuesday that he would be replaced as secretary of state by CIA Director Mike Pompeo.

In Moscow, the Foreign Ministry said it presented the British ambassador with “a strong protest over the unfounded accusations leveled at Russia by British authorities” and stressed that “Moscow would not respond to London’s ultimatum until the Russian side is provided with samples of the chemical substance.”

And it promised that Russia would retaliate if sanctions are imposed. “Any threats will not remain unanswered,” the ministry said in a statement. “The British side should be aware of that.”

May said the use of Novichok, which is believed to be unique to Russia, pointed to Moscow’s complicity in the poisoning of Skripal, a former Russian double agent, and his daughter in Salisbury, about 90 miles southwest of London. Both remain comatose.

In Moscow, Lavrov denied that Russia had anything to do with Skripal’s poisoning and reiterated Moscow’s willingness to cooperate if information related to the nature of the chemical agent was shared with Russia.

Lavrov said Britain has an obligation to share forensic data under the Chemical Weapons Convention.

“Before delivering ultimatums to report to the British government within 24 hours,” Lavrov said at a news conference in Moscow, “it is better to comply with your own obligations under international law — in this case, the Convention on the Prohibition of Chemical Weapons.”

Russia’s representative to the Organization for the Prohibition of Chemical Weapons, Alexander Shulgin, told a meeting of the group’s executive board Tuesday that London’s allegations of Russian involvement were unfounded and unacceptable, the Interfax news agency reported. He called on Britain to turn over samples to the organization for independent laboratory analysis.

His British counterpart, Peter Wilson, said investigators had concluded that Russia was responsible based on a positive identification of the chemical agents, a history that links Novichok to Russia, Russia’s “record of conducting state-sponsored assassinations” and a British assessment that Russia considers some defectors to be “legitimate targets for assassination.”

“This attempted murder, using a weapons-grade nerve agent in a British city, was not just a crime against the Skripals. It was an indiscriminate and reckless act against the United Kingdom, which put the lives of innocent civilians at risk,” Wilson said.

Russia, he said, had either engaged in a direct attack against Britain or lost control of the nerve agent it developed.

May had given Russia one day to provide an explanation. She promised to return to Parliament on Wednesday with a plan for specific action.

in her remarks, May described a “reckless” and “indiscriminate” attack against Skripal, 66, and his daughter Yulia, 33. A police officer also remains hospitalized.

On the Russian Foreign Ministry’s verified Twitter account, the posts carried a characteristically flippant and sarcastic tone. It launched a hashtag, #HighlyLikelyRussia, and portrayed May’s ultimatum as part of broader anti-Russian hysteria plaguing Western discourse.

“Sincere thanks to Mrs. May for #HighlyLikelyRussia,” a tweet read. The post included a video of recent intense snowfall in Britain, mockingly suggesting that Russia was to blame for the weather. The video concludes with an image of a penguin, and signs off with “at least penguin enjoys it.”

Konstantin Kosachev, the head of the Foreign Affairs Committee in the Federation Council, Russia’s upper chamber of parliament, wrote on Facebook that May’s accusations were “despicable and unacceptable.”

“For Britain, the Queen of Courts, this is a complete degradation,” Kosachev wrote. “The accused has to provide the proof, not the court or the prosecutor, without being given access either to the evidence or the trial itself.”

Vyacheslav Volodin, speaker of the Russian State Duma, the lower house of parliament, asserted that the British allegations were part of a planned effort to interfere in Sunday’s Russian presidential election.

“It is during this period that these events unfold in order to try to discredit Russia in the eyes [of] the international community, in order to create this unfavorable background in the conduct of the election campaign,” he said, according to Interfax. Citing “this interference in our elections,” he added: “The form chosen is the most cynical, when the health of citizens is put at risk. . . . Once again I want to say that Britain is responsible for this.”

His counterpart in the Federation Council, Valentina Matviyenko, echoed his words.

“In Russia, a very important political campaign is underway on preparing for the presidential election,” Matviyenko said. “This is another fake aimed at whipping up another round of the Russophobic campaign.”

While most of the reactions have so far avoided the topic of Novichok, the nerve agent, other members of Russia’s Federation Council addressed the accusations head-on.

Council member Igor Morozov, a veteran of the Russian security services, told the RIA Novosti news agency that “Russia has not only stopped producing nerve agents, including Novichok, but also completely destroyed all of its stockpiles.”

However, he also said it would be “dangerous but possible” to secretly produce Novichok, although that would require special facilities and technicians.

Last year, the Organization for the Prohibition of Chemical Weapons announced the destruction of Russia’s final batch of declared chemical weapons. However, Russian scientists who blew the whistle on Novichok’s existence in 1992 claimed at the time that the nerve agent was designed specifically to skirt chemical weapons conventions.