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U.S. Ambassador to the United Nations Nikki Haley said on Wednesday the United States believes Russia is responsible for the attempted assassination of a former Russian double agent and his daughter in Britain — and the U.N. Security Council should hold the Kremlin "accountable."

"The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent," Haley said at a Security Council meeting in New York.

Haley said the United States stood in "absolute solidarity" with Britain after the country expelled 23 Russian diplomats in response to the chemical attack last week on the ex-spy, Sergei Skripal, and his daughter, Yulia.

Related: U.K. to expel 23 diplomats in retaliation for spy poisoning

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She called on the U.N. to take action action, saying that the "credibility of this Council will not survive if we fail to hold Russia accountable."

"If we don't take immediate, concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used," Haley said, referring to the English city where Skripal and his daughter were discovered unconscious on a bench.

Russia has denied any involvement in the assassination attempt.

Haley's remarks echoed those of Rex Tillerson, the ousted secretary of state who this week called the nerve agent attack "a really egregious act" that appears to have "clearly" come from Russia.

President Donald Trump, for his part, has not pinned the blame on the Kremlin, saying this week that "as soon as we get the facts straight," the United States would "condemn Russia or whoever it may be."

British Prime Minister Theresa May, however, has blasted Russia in the wake of the March 4 attack, saying it was "highly likely" that the government of Vladimir Putin was responsible.

"There is no alternative conclusion other than that the Russian State was culpable for the attempted murder of Mr. Skripal and his daughter — and for threatening the lives of other British citizens in Salisbury," May said on Wednesday.

Related: How can U.K. respond to Russia-linked spy attack?

Britain moved to expel the Russia diplomats on Wednesday after Moscow ignored a midnight deadline to explain how its nerve weapon was used in the attack. The expulsions — the largest in three decades — "will fundamentally degrade Russian intelligence capabilities in the U.K. for years to come," May said.

Skripal, the 66-year-old ex-spy targeted with the nerve agent, is a former Russian military officer who was sentenced to 13 years in prison after being convicted in 2006 of spying for Britain. He passed the identity of dozens of spies to the U.K.'s foreign intelligence agency, according to news reports.

He was freed in 2010 as part of a U.S.-Russian spy swap that also included spy Anna Chapman, who was arrested in New York earlier that year.