Trump told May that the US is with the UK 'all the way' over the Sergei Skripal poisoning

President Donald Trump has told British Prime Minister Theresa May that the US is with the UK "all the way" over the poisoning of ex-spy Sergei Skripal and that Russia must provide "unambiguous answers" over the use of nerve agent in the attack, according to a Number 10 spokesperson.

The UK prime minister spoke to the US president on Tuesday about the incident, which has left Skripal and his daughter Yulia critically ill and sent relations between the UK and Russia plummeting.

While May has said it is "highly likely" that the Russian government is behind the nerve gas attack, the White House has been more reticent about attribution. In a press briefing on Monday, White House press secretary Sarah Huckabee Sanders declined to say whether the US endorsed the UK's view that Russia is responsible or to condemn Russia by name.

"As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be," Trump said earlier on Tuesday.