UK analysis of samples from the Syria chemical attack has detected sarin or a sarin-like substance, Britain's UN ambassador has said.
Matthew Rycroft told the Security Council: "The United Kingdom therefore shares the US assessment that it is highly likely that the regime was responsible for a sarin attack on Khan Sheikhoun on 4 April."
But Russia's deputy ambassador to the UN, Vladimir Safronkov, said he was "amazed at this conclusion".
"No one has yet visited the site. How do you know that?" he added.
The deadly toxic gas attack has been widely blamed on Syria's President Assad and prompted the US to retaliate with a cruise missile strike on one of the country's airbases.
Mr Rycroft was speaking during a UN Security Council meeting to discuss a resolution put forward by the US, UK and France, which, as expected, was vetoed by Russia.
The resolution would have demanded the Syrian government cooperate with an investigation into the chemical attack in the rebel-held town of Khan Sheikhun.
The Syrian regime and its backers, including Russia, have denied Mr Assad was responsible.
They have said the attack which killed more than 80 people was either a rebel provocation or the result of Syrian planes hitting a rebel weapons factory.
It comes as US secretary of state Rex Tillerson met Russian foreign minister Sergei Lavrov and later President Vladimir Putin for talks over the escalating crisis .