(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."