The White House says it “stands in solidarity” with the “its closest ally” the UK and supports its decision to expel 23 Russian diplomats.
PM Theresa May said the diplomats would be expelled after Moscow refused to explain how a Russian-made nerve agent was used on a former spy in the UK.
Moscow continues to deny any involvement in the poisoning.
US President Trump’s spokeswoman accused Russia of undermining the security of countries worldwide.
BBC’s North American editor Jon Sopel said the White House statement was notable in the unqualified support they offer Theresa May – but significant too in the way President Trump is prepared to talk about Russia.
He said this is language not heard from the White House before.
In the statement, White House press secretary Sarah Sanders said the US wanted to ensure “this kind of abhorrent attack does not happen again”.
She described the expulsion of Russian diplomats from Britain as “a just response”
“This latest action by Russia fits into a pattern of behaviour 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,” she said.
Former spy Sergei Skripal, 66, and his daughter, Yulia Skripal, 33, remain critically ill in hospital after being found slumped on a bench in Salisbury, Wiltshire, on 4 March.
Det Sgt Nick Bailey fell ill while responding to the incident, and is in a serious but stable condition, but is thought to be improving.
Mr Skripal came to the UK in 2010 as part of a “spy swap” after he had been convicted by Russia of passing information to MI6.
The White House statement echoed earlier comments made by the US ambassador to the UN, Nikki Haley, who cited the “special relationship” between the two countries and said the US would “always be there” for the UK.
Also addressing the UN Security Council, Britain’s deputy UN ambassador, Jonathan Allen, said he had heard threats from Russia but that the UK would not be deterred.
“We will stand by the values which are shared by the overwhelming majority of those in this council in this United Nations and we ask you today, to stand by us,” he added.
In response, the Russian ambassador to the UN, Vasily Nebenzya, denied Moscow’s involvement in the attack and demanded “material proof” from Britain to support its charge.
The expulsion of the diplomats is the largest since 31 were ordered out in 1985 after double agent Oleg Gordievsky defected.
Russian foreign minister Sergei Lavrov said Moscow would co-operate in the case if it received a formal request for clarification from the UK under the Chemical Weapons Convention, which sets a 10-day time limit for a response.
Moscow refused to meet the UK’s deadline to co-operate, prompting Mrs May to announce the diplomats’ expulsion and other measures intended to send a “clear message” to Russia.
- Increasing checks on private flights, customs and freight
- Freezing Russian state assets where there is evidence they may be used to threaten the life or property of UK nationals or residents
- Ministers and the Royal Family boycotting the Fifa World Cup in Russia later this year
- Suspending all planned high-level bilateral contacts between the UK and Russia
- Plans to consider new laws to increase defences against “hostile state activity”
In a statement to MPs Mrs May said that Russia had provided “no explanation” as to how the nerve agent came to be used in the UK, describing Moscow’s response as one of “sarcasm, contempt and defiance”.
Russia’s foreign ministry said the statement was “an unprecedentedly crude provocation” and that the UK government had “seriously aggravated” relations by announcing a “whole set of hostile measures”.