The White House says it “stands in solidarity” with “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 attack.
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 it offers Theresa May.
He said it was also significant because of the way President Trump was prepared to talk about Russia – using language not been heard from the White House before.-BBC/UNS