A Downing Street spokesperson said:

The Prime Minister spoke to President Trump earlier this afternoon to update him on the ongoing investigation into the Salisbury incident.

The Prime Minister set out the conclusion reached by the UK Government that it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal.

President Trump said the US was with the UK all the way, agreeing that the Russian Government must provide unambiguous answers as to how this nerve agent came to be used.

https://www.gov.uk/government/news/pm-call-with-president-trump-13-march-2018