President Donald Trump has backed UK prime minister Theresa May's demand that Russia must provide "unambiguous answers" over last week's poisoning of an ex-Russian spy and his daughter in the English city of Salisbury.
May's office says Trump and the British leader spoke by phone on Tuesday afternoon and quoted Trump as being "with the UK all the way" in reiterating Britain's view that "it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal."
May has demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out measures Britain will adopt if no credible response is received by the end of Tuesday.
But Russia says it will refuse to meet May's midnight deadline unless Britain agrees to send Moscow samples of the nerve agent used to poison Sergei Skripal.
The country's embassy in the UK fired off a salvo of seven tweets in which it said Britain must comply with the Chemical Weapons Convention for a joint investigation and warned the threat of sanctions would "meet with a response".
Britain's ambassador to Russia was also summoned by Moscow and told by first deputy foreign minister Vladimir Titov that the Kremlin "strongly protested" the accusations, the embassy said.
The White House said in a statement on Tuesday: "President Trump agreed with Prime Minister May that the Government of the Russian Federation must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom,
"The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms."
Earlier, speaking to reporters, Trump said: "It sounds to me like they believe it was Russia and I would certainly take that finding as fact.
"As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be."