On Tuesday, one senior State Department official, speaking on background, supported the UK's own decision to kick out 23 Russian diplomats: "We stand in solidarity with our Allies in the United Kingdom, and we fully supported the UK's decision to expel diplomats and take other actions to respond".
"The exact number is now being discussed with our European partners and will be announced on Monday".
Welcoming the support from the other 27 European Union leaders, Mrs May said it was right they were standing together.
'We have seen the Russian state deploy an extensive disinformation campaign around the Salisbury incident, ' a British official told the Times.
Latvia's security services regularly mention in their public reports that Russia's intelligence and security services pose the greatest threat to national interest, according to the Baltic news agency BNS.
The former Polish prime minister, who went out of his way to note that the show of European Union unity was taking place "despite tough Brexit negotiations", added: "I expect that a number of member states will take additional measures towards Russian Federation on Monday". But in traditional European Union style, they agreed to the latest text after extracting concessions on other issues - in this case, wording critical of Turkey's actions in the Eastern Mediterranean and the Aegean, the site of tensions over drilling for gas.
Both of the Skripals remain in a serious but stable condition in hospital.
Russia has denied responsibility for the March 4 attack on former Russian spy Sergei Skripal and his daughter, the first known offensive use of a nerve toxin in Europe since the Second World War.
But German Chancellor Angela Merkel said evidence of Russian culpability presented by British Prime Minister Theresa May was "very solidly based" and promised to take further measures.
"We don't agree with this and we repeat again that Russian Federation absolutely definitely has nothing to do with the Skripal case".
She said: "I welcome the fact that the EU Council has agreed with the United Kingdom Government's assessment that it is highly likely that Russian Federation was responsible for the attempted murder that took place on the streets of Salisbury and that there is no plausible alternative explanation".
At a Brussels summit, May shared information about why Britain is convinced Moscow was behind the nerve-agent attack.
The EU states said they would "coordinate on the consequences to be drawn in the light of the answers provided by the Russian authorities".
"We are determined to react together, with the language we used here, but also possibly through additional measures", Merkel said on March 22. "We are not going to randomly expel people who are genuine diplomats", Varadkar told reporters.
Welcoming the solidarity she secured from the summit, May told reporters on leaving: "The threat from Russian Federation is one that respects no borders and I think it is clear that Russian Federation is challenging the values we share as Europeans and it is right that we stand together in defence of those values".
But she said that "we, Germany and France, agreed that such reactions in addition to the recall of the (EU) ambassador are necessary".