Russian claims that a release of chemicals that killed and injured dozens of civilians in northern Syria came from rebel weapons on the ground have been rejected.
Britain’s foreign secretary, a rebel commander and a weapons expert all said the evidence pointed to an attack by Syrian government forces.
Damascus denies its forces launched a chemical weapons attack.
The issue is overshadowing a conference on Syria in Brussels.
Seventy donor nations are discussing aid efforts in the war-ravaged country.
The United Nations Security Council is due to hold emergency talks later following the release of gas in Khan Sheikhoun in Idlib province which – according to UK-based monitoring group the Syrian Observatory for Human Rights – killed 72 people, including 20 children.