Russia's President Vladimir Putin (L) and US President Donald Trump talking during a bilateral meeting on the sidelines of the G20 summit in Hamburg.

President Donald Trump is considering the expulsion of some Russian diplomats in the United States in solidarity with Britain over the poisoning of a former Russian spy in England, a source familiar with the situation said on Sunday.

The move may be contingent on how European capitals respond to the nerve agent attack, the source said.

An announcement of the U.S. decision could be made as early as Monday, the source said.

European Union member states agreed on Friday to take additional punitive measures against Russia for the nerve-agent attack on the former Russian spy Sergei Skripal and his daughter Yulia, who have been critical in hospital since they were found unconscious on March 4 on a bench in the city of Salisbury.

White House spokesman Raj Shah said the United States is considering how to respond but would not provide details.

"The United States stands firmly with the United Kingdom in condemning Russia's outrageous action. The President is always considering options to hold Russia accountable in response to its malign activities. We have no announcements at this time," he said.

Moscow has denied responsibility for the attack and has retaliated against Britain's move to expel 23 Russians by ordering out the same number of Britons.

The United States joined with Britain in blaming Russia for the attack.

Trump has sought to improve ties with Russian President Vladimir Putin, and last week in a phone call with Putin congratulated him his disputed re-election victory.