The British government has confirmed sarin "or a sarin like substance" has been detected in samples taken from the Syrian province where more than 80 people were killed in a suspected chemical attack last week.
It is "highly likely" the Assad regime is responsible, said UK ambassador to the UN Matthew Rycroft.
The news was delivered to a meeting of the UN Security Council on Wednesday, where Russia said it would vote against a Western-drafted resolution condemning the reported use of chemical weapons and calling for an international inquiry
Heated scenes erupted when Mr Rycroft accused Moscow of siding with "a murderous, barbaric criminal, rather than with their international peers."
Russia's deputy United Nations ambassador Vladimir Safronkov, who demanded Mr Rycroft look at him while he was speaking, responded: "I cannot accept that you insult Russia."
It's not the first time the pair have clashed over the issue in the past week.