A senior European diplomat said the US has been considering the expulsion of 20 or more Russian diplomats in solidarity with Britain. The decision rests with the White House, the official said, but close allies have urged the Trump administration to take the symbolic but dramatic step as a signal that the assassination attempt was a test of Western resolve that must be answered, the diplomat said.

Officials, including two senior White House officials, spoke on the condition of anonymity to describe a sensitive decision that hadn't been finalised.

Republicans and Democrats in Congress have pressed the White House to move more aggressively against Russia in the wake of its alleged interference in the 2016 election. In August, lawmakers passed legislation limiting the president's ability to lift sanctions imposed on Russia in response to its military intervention in Ukraine.

Since coming to power, the Trump administration has taken action against Moscow, closing its consulate in San Francisco and approving lethal aid to Ukraine in its battle with pro-Russian separatists, a move the Obama administration opposed.

Earlier this month, the administration imposed fresh sanctions on Russian government hackers and spy agencies for interfering in the 2016 election and for a global cyberattack that officials have described as the most destructive and costly in history.

Trump, however, has refrained from criticising Russian President Vladimir Putin, including in a phone call last week after Russia's disputed presidential elections.

In advance of the call, the president's staff included a section in his briefing materials in all-capital letters stating "DO NOT CONGRATULATE" and recommending that he raise the poisoning of the former Russian spy. The president chose to congratulate Putin and did not raise the poisoning incident, US officials confirmed.

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A number of European countries - close to 20 - are expected to expel Russian officials and take other measures as early as Monday. The countries include the Baltic nations of Latvia, Lithuania and Estonia, as well as Sweden, Poland, Denmark, the Czech Republic, France and the Netherlands.

"It's a big test for the UK, especially in the post-Brexit context, to see how many real allies they have," said a European official, who spoke on the condition of anonymity because he was not authorised to talk on the record.

On Saturday, White House spokesman Raj Shah said 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."

Washington Post