Donald Trump. Win McNamee/Getty Images 
 
 Hours after British Prime Minister Theresa May announced the UK will expel 23 Russian diplomats as punishment over the nerve agent attack against a former spy on British soil, President Donald Trump has yet to weigh in. 
 
 "This will be the single biggest expulsion for over thirty years and it reflects the fact that this is not the first time that the Russian State has acted against our country," May said. "For those who wish to do us harm, my message is clear. You are not welcome here." 
 
 May also said that Russians under suspicious would have their assets frozen, and that the British Royal Family and government ministers would not attend the 2018 World Cup in Russia. 
 
 Meanwhile, Trump's silence on the matter since he briefly acknowledged the chemical attack Tuesday has been deafening to former US diplomats, many of whom wonder why the US hasn't been steadfast since the beginning in backing its closest international ally on the matter. 
 
 May announced Monday that it was "highly likely" the Russian government ordered an assassination attempt against Sergei Skripal, a former Russian military intelligence officer who later became a double agent for the UK, in Salisbury, England earlier this month. 
 
 Skripal and his 33-year-old daughter, Yulia, were both hospitalized and remain in critical condition after being exposed to Novichok, a nerve agent developed by the Soviet Union in the 1970s and 1980s. 
 
 Shortly after May said Russia was the likely culprit in Skripal's case, the White House called the attack "reckless, indiscriminate, and irresponsible" but declined to specifically name Russia, saying the details still needed to be sorted out. That evening, Secretary of State Rex Tillerson took a more hardline stance, saying Russia was "clearly" behind the attack. Tuesday morning, Trump fired Tillerson via Twitter. 
 
 While answering questions about Tillerson's firing, Trump said of the chemical attack on Skripal, "As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be." Referring to the UK's findings, he added, "It sounds to me like they believe it was Russia, and I would certainly take that finding as fact." 
 
 On Wednesday, after May announced the expulsion of the 23 Russian diplomats — the largest number ejected by the UK since the Cold War —multiple US officials, including the US ambassador to the UN and White House staff, said the US stands in solidarity with the UK. 
 
 However, the president himself has not come out and publicly made that clear since his brief mention on Tuesday. 
 
 'Judgment day for Donald Trump' 
 
 President Donald Trump and Britain's Prime Minister Theresa May react during a ceremony at the new NATO headquarters before the start of a summit in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann 
 
 "Judgment day for Donald Trump," R. Nicholas Burns, a former US ambassador to NATO, tweeted after the UK announced the measures it was taking to penalize Russia. "Will he support Britain unequivocally on the nerve agent attack? Back #NATO sanctions? Finally criticize Putin? Act like a leader of the West? 
 
 Richard Kauzlarich, the former deputy assistant secretary of state in the Bureau of European Affairs, said the White House should have backed the British from the beginning. 
 
 "When you have the British come out as clearly and decisively as they did about who was responsible, the logic and gravity of the situation would require the president to say something in solidarity," he said. 
 
 "It should be almost automatic, especially with an ally this close to the US." 
 
 Edward Price, the former senior director of the National Security Council under President Barack Obama, echoed that view. 
 
 "For years, we heard from voices on the political right that America couldn't possibly confront its adversaries without first clearly naming them," he said. "And [on Monday], we heard the White House Press Secretary condemn the act but very deliberately skirt the actor, which, by all accounts, appears to be Moscow in this case." 
 
 He added that not only did the US effectively hang the UK "out to dry" by not staying in lockstep with the ally from the start, it was also signaling to Russian President Vladimir Putin that "he is free to act with impunity, including by carrying out deadly acts in the UK." 
 
 The Skripal attack is perhaps the most high-profile case of its kind since former Russian spy Alexander Litvinenko, a vocal Putin critic, was murdered in the UK in November 2006. A public inquiry into Litvinenko's death found in 2016 that Russian intelligence officials were responsible for the assassination, and that Putin was "probably" behind it. 
 
 Moscow scoffed at the UK's latest accusations linking it to the Skripals' attempted assassination, dismissing them as a "circus show." After the UK announced the expulsion of nearly two dozen Russian diplomats on Wednesday, Russia called it a "very serious provocation" and a "hostile action." 
 
 Meanwhile, Trump's silence since May's remarks on Wednesday morning mark yet another incident in which the president, "for whatever reason, is not prepared to put Russia on the spot," Kauzlarich said. "And Russia will draw the conclusion that anybody would: that if this president, for reasons no one can understand, will not criticize Russia on something this blatant, what will it take?" 
 
 "All of us who have been around the block find it bizarre that we would not be 100% behind the Brits on this," he added. "They've earned it. Russia hasn't." ||||| Breaking News Emails Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings. 
 
 / Updated By Daniel Arkin 
 
 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 
 
 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. ||||| (CNN) US Ambassador to the United Nations Nikki Haley said Wednesday the Trump administration "stands in absolute solidarity with Great Britain" following a nerve agent attack against a Russian double agent and his daughter in the English city of Salisbury last week. 
 
 In the strongest statement yet from the US administration on the affair, Haley said Washington shared the UK's assessment that the Russian state was behind the poisoning and demanded a firm international response. 
 
 "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 in her remarks at a UN Security Council emergency session, blasting the Russian government for flouting international law. 
 
 "If we don't take immediate concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used," said Haley. "They could be used here in New York or in cities of any country that sits on this council." 
 
 The United Kingdom believes Russia was behind the attempted murders of Sergei Skripal and his daughter Yulia because the nerve agent used, Novichok , was developed in the Soviet Union and could not be replicated by non-state actors. London announced Wednesday it would expel 23 Russian diplomats after Moscow failed to meet a UK deadline to give a "credible response." 
 
 Russia has dismissed the accusations as "fairy tales" and denied any involvement in the attack which landed the Skripals, along with a British police officer, in the hospital. 
 
 Moscow's ambassador to the UN, Vassily Nebenzia, even suggested the UK might have been responsible for the attack in an attempt to smear Russia. "In the Russian Federation, no scientific research or development work under the title Novichok were carried out," he told the Security Council. 
 
 "The most probable source origin this chemical are the countries which have since the end of the 90s been carrying... out intensive research on these kinds of weapons, including the UK." 
 
 But Haley laid the blame firmly at Russia's door. Highlighting Moscow's support of the Assad regime in Syria following that government's use of chemical weapons against civilians, Haley told fellow diplomats the world had reached "a defining moment." 
 
 "Time and time again, members states say they oppose the use of chemical weapons under any circumstance," said Haley. "Now one member stands accused of using chemical weapons on the sovereign soil of another member. The credibility of this council will not survive if we fail to hold Russia accountable." 
 
 Presenting Britain's case, Deputy UK Ambassador to the UN Jonathan Allen called the attack "an unlawful use of force" and invited representatives from the Organization for the Prohibition of Chemical Weapons to conduct an independent investigation of the incident. 
 
 Nebenzia, Russia's envoy, reaffirmed his government's denial of involvement in a lengthy and colorful response. 
 
 He called on the UK government to offer proof that Novichok was used and that Russia was responsible, and slammed UK Prime Minister Theresa May for making "completely irresponsible statements" and "threats." 
 
 In questioning London's allegations, Nebenzia cited the English fictional detective Sherlock Holmes and his "hapless" Scotland Yard counterpart, Inspector Lestrade. 
 
 "Lestrade latches on to something that is on the surface of a crime and is in a hurry to provide banal conclusions only to be overturned by Sherlock Holmes, who always finds what is behind the crime," Nebenzia said. "I do think we could all stand to benefit from having a Sherlock Holmes with us today." 
 
 With apparent sarcasm, Nebenzia also suggested the UK government should look inward to determine why Russian nationals in the country so often find themselves in mortal peril, and mocked Haley's credibility as "an experienced chemist." 
 
 The White House later backed up Haley's comments at the UN, saying it shared the British assessment of Russia's culpability in the Salisbury attack. "The United States stands in solidarity with its closest ally, the United Kingdom," the statement read. 
 
 "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."
– Nikki Haley came out strong against Russia and in support of the United Kingdom on Wednesday following the nerve agent attack that left a former Russian spy and his adult daughter in critical condition in England, CNN reports. "The United States believes that Russia is responsible for the attack," the US ambassador to the UN told the Security Council, adding the White House "stands in absolute solidarity with Great Britain." "Time and time again, member states say they oppose the use of chemical weapons under any circumstance," Haley said. "Now one member stands accused of using chemical weapons on the sovereign soil of another member. The credibility of this council will not survive if we fail to hold Russia accountable." It was by far the most forceful statement out of the Trump administration. Rex Tillerson had said Russia was "clearly" responsible for the attack on Sergei Skripal and daughter Yulia but was shortly fired by Trump, NBC News reports. When asked about Tillerson's firing Tuesday, Trump said "as soon as we get the facts straight" the US would "condemn Russia or whoever it may be." According to Business Insider, former US diplomats have expressed confusion about why the White House hasn't done more to back its ally Great Britain, which has expelled 23 Russian diplomats. Nicholas Burns calls it "judgment day" for Trump. "Will he support Britain unequivocally on the nerve agent attack?" the former US ambassador to NATO says. "Back NATO sanctions? Finally criticize Putin? Act like a leader of the West?"