Russia denies any involvement in the March 4 attack on Sergei Skripal, 66, and his daughter Yulia, 33, who are both critically ill in hospital.
The Prime Minister has suggested Moscow was behind the attack or has lost control of its nerve agent stocks.
Speaking a special meeting of the UN Security Council called to discuss the incident, Mrs Haley said the United States agreed with Theresa May that Russia was responsible for the nerve gas attack on a Russian double agent and his daughter in Salisbury.
And she said her country stands “in absolute solidarity” with the UK – while urging the UN to “hold Russia to account” for its actions.
Ms Haley: “If we don’t take immediate concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used.
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“They could be used here in New York or in cities of any country that sits on this council.
“This is a defining moment.
“Time and time again member states say they oppose the use of chemical weapons under any circumstances.
“Now one member stands accused of using chemical weapons on the sovereign soil of another.
“The credibility of this council will to survive if we fail to hold Russia to account.”
The 46-year-old former senator for South Carolina, who was nominated by US President Donald Trump two weeks after his 2016 election victory, added: “Let me make one thing clear from the very beginning, the United States stands in absolute solidarity with Great Britain.
“The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent. Dozens of civilians and first responders were also exposed.
“No two nations enjoy a stronger bond than that of the United States and the United Kingdom.
“Ours is truly a special relationship. When our friends in Great Britain face a challenge, the United States will always be there for them. Always.”
Sergei Skripal and his 33-year-old daughter Yulia remain in hospital in a critical but stable condition, ten days after the incident, in which they were apparently attacked using a nerve gas known as novichok.
Police officer Nick Bailey, the first attender on the scene, is also seriously ill as a result of the incident.
Mrs Haley’s words come on the same day Mrs May announced that 23 Russian diplomats were being expelled in retaliation.
In making her statement, Mrs Haley went significantly further than US President Donald Trump when he was questioned about the matter yesterday, when he commented: “It sounds to me like it would be Russia based on all of the evidence they have.
“I don’t know if they’ve come to a conclusion. As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be.”