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.
US President Donald Trump’s spokeswoman accused Russia of undermining the security of countries worldwide.
Moscow denies responsibility and says it will expel UK diplomats “soon”.
According to BBC, Foreign Minister Sergei Lavrov told Russian media the expulsions would “definitely” happen and reportedly referred to the UK’s allegations as “absolutely boorish”.
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.