United States Ambassador to the United Nations (U.N.) Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE on Wednesday accused Russia of flagrantly and aggressively using a nerve agent to poison a former Russian spy in the United Kingdom, and called on Moscow to "come clean" about its chemical weapons program.

“The United States believes that Russia is responsible for two people in the United Kingdom using a military-grade nerve agent,” Haley said at the U.N. in New York.

"Russia must fully cooperate with the U.K.'s investigation and come clean about its own chemical weapons program," she added.

Haley's comments were the strongest to date from a Trump administration official regarding the poisoning of Sergei Skripal and his daughter, Yulia, with a nerve agent.

U.K. officials have blamed Russia for the poisoning, which has left the former Russian spy and his daughter critically ill in the hospital.

Haley on Wednesday also blamed Moscow for chemical strikes allegedly perpetrated by the Syrian government in rebel-held enclaves of the war-torn country, saying that Russia’s failure to oversee the disposal of Syria’s chemical weapons ultimately facilitated the attacks.

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White House deputy press secretary Raj Shah said Wednesday that it "certainly appears" that Russia was behind the poisoning in the U.K.

"The president did tell the prime minister on the phone yesterday the United States does stand 100 percent with our ally [the] U.K. on this matter," he said.

His comments were followed by the strongest statement yet from the White House regarding Russia's role in the poisoning.

"The United States shares the United Kingdom’s assessment that Russia is responsible for the reckless nerve agent attack on a British citizen and his daughter, and we support the United Kingdom’s decision to expel Russian diplomats as a just response," said press secretary Sarah Huckabee Sanders later on Wednesday. "This latest action by Russia fits into a pattern of behavior in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes."

On Monday, Sanders had said that, while the U.S. stood behind the U.K., the details of the case were still being worked out.

British Prime Minister Theresa May moved on Wednesday to expel 23 Russian diplomats from the U.K. after Moscow blew past a U.K.-imposed deadline to explain the poisoning of the Skripals. The Russians have denied any role in the attack.

“They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance,” May said, referring to the Russians.

"The United States is working together with our allies and partners to ensure that this kind of abhorrent attack does not happen again," Sanders said in the statement.

–– Jonathan Easley contributed to this report

Updated: 6:38 p.m.