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Pavel Golovkin
Russian diplomats ordered to leave Britain as part of a standoff over a nerve agent attack on British soil, leave the gate of Vnukovo 2 government airport outside Moscow, Russia, Tuesday, March 20, 2018. Nearly two dozen Russian diplomats expelled by Britain over the poisoning of an ex-spy arrived home Tuesday, while a scientist involved in the creation of the nerve agent said it could be manufactured by other countries. (AP Photo/Pavel Golovkin)

MOSCOW — British Foreign Secretary Boris Johnson insisted Wednesday that Moscow was responsible for a nerve-agent attack on an ex-spy on British soil, while a senior Russian diplomat challenged London to proof its claims, accusing it of "hiding facts" or staging the attack itself.

The war of words came as a diplomatic crisis deepened over the March 4 poisoning of Russian ex-spy Sergei Skripal and his daughter in the English city of Salisbury. Britain maintains Russia was behind the attack, using a military-grade nerve agent known as Novichok, which left the father and daughter in critical condition. Moscow has fiercely denied involvement.

Johnson on Wednesday repeated Britain's position that responsibility for the attack leads "back to the Russian state and those at the top." He added that the attack had prompted "a mountain of disgust globally," and that he had been pleasantly surprised "at the strength of the solidarity that there is with the U.K."

But Vladimir Yermakov, deputy head of the Russian Foreign Ministry's department for non-proliferation, scoffed at expressions of Western solidarity, saying they are meaningless in the absence of proof.

"Have they fallen under the spell of the 'Strike Back' television series?" Yermakov told a briefing with foreign diplomats, referring to the British-American action TV series featuring a plot involving Novichok.

Yermakov also accused Britain of "hiding facts" and warned that key evidence might "disappear." Casting doubt on Britain's credibility, he said experts would agree that "the use of a military-grade nerve agent would lead to numerous fatalities right on the spot, but we have seen quite a different picture in Salisbury."

"Two versions are possible: Either the British government has failed to ensure protection from such terror attack or have staged it themselves, directly or indirectly," he said.

British officials have previously dismissed such Russian allegations as "nonsense."

Britain and Russia have expelled 23 of each other's diplomats in a tit-for-tat response to the attack, and Britain is seeking to rally allies for new sanctions against Moscow.

Yermakov urged Britain to "come forward and open all the data," and warned that Moscow would not recognize any probe if it doesn't see the evidence.

He added that Britain's use of the name Novichok — never used by the agent's Soviet designers — was part of efforts to put the blame on Russia.

He insisted that Russia "has nothing to do with (the poisoning) whatsoever" because it "does not benefit us in any way."

Russia previously claimed it had no motive to kill Skripal, who was convicted of spying for Britain but released in a 2010 spy swap. Moscow has also insisted that it had completed the destruction of its chemical arsenals last year under international oversight.

Meanwhile, a Russian scientist involved in designing Novichok said that lab tests can clearly determine its origin.

Vladimir Uglev told The Bell online news portal that the name Novichok common in the West wasn't used by its Soviet designers. He said in an interview posted late Tuesday that numerous variants of such agents were developed by the Soviets in 1972-1988.

Uglev said British chemical experts could have created their versions of such agents. He noted that blood samples could show who produced the agent that poisoned the victims.

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Another Russian scientist involved in the agent's creation, Leonid Rink, told the state RIA Novosti news agency Tuesday that Britain and others could easily synthesize Novichok after chemical expert Vil Mirzayanov emigrated to the U.S. and revealed its formula.

Gen. Igor Kirillov of the Russian Defense Ministry on Wednesday called the publication of Mirzayanov's book "complicity to terrorism."

Kirillov also accused Western nations of "exploiting all possible methods to discredit Russia." Yermakov hinted that the poisoning "may have been directed from overseas," adding that the United States still has a sizeable stockpile of chemical weapons.

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Jill Lawless contributed from London.