The US has backed Britain in condemning the nerve agent attack on Sergei Skripal and his daughter, Yulia.
The White House dubbed it “an outrage” and said it stood by the UK.
Press secretary Sarah Huckabee Sanders said: “We’ve been monitoring the incident closely, taking it very seriously.
“The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage.
“The attack was reckless, indiscriminate and irresponsible.
“We offer the fullest condemnation and we extend our sympathy to the victims and their families and our support to the UK Government.
“We stand by our closest ally and the special relationship that we have.”
However, White House reporters said officials repeatedly declined to echo Theresa May’s view that Russia was behind or complicit in the attack.
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“Sanders said repeatedly that the United States stands with its ally, Britain, but it didn’t join its conclusions about Russia’s involvement yet,” said Aaron Blake, of The Washington Post.
Mr Skripal and his daughter are still being treated in hospital after falling ill in Salisbury.