US President Donald Trump has ordered the expulsion of 60 alleged Russian spies from the United States in response to a nerve agent attack on an ex-spy in the English city of Salisbury.

White House press secretary Sarah Sanders said the move, part of globally coordinated retaliation against Moscow, was taken "in response to Russia's use of a military-grade chemical weapon on the soil of the United Kingdom."

Officials said that 48 "known intelligence officers" posted to the United States and 12 more at the Russian mission to the United Nations now have seven days to leave the country.

Trump has also ordered the closure of the consulate in Seattle, which officials said was the focus of spying efforts against the nearby Kitsap submarine base and Boeing.

One senior administration official said that there were still more that 40 known Russian intelligence officers operating in the United States, but Moscow's "collection capabilities" would be "significantly" affected by the move.

On March 4, former spy Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious, slumped on a bench outside a shopping center in Salisbury.

After initial mixed messages about who was to blame, Washington now agrees with Britain's assessment Russia was behind the attack.

The message, a second senior administration said, is "when you attack our friends you will face consequences."