Human Rights Watch says Syrian forces were "almost certainly responsible" for the chemical weapons attack last month that killed hundreds of people.
The group said in a new report Tuesday that the type of rockets used, photos and videos from the attack sites, and interviews with victims and doctors all implicate government forces.
The Syrian government has denied it was behind the August 21 attacks, accusing rebel fighters of deploying the chemical weapons in the suburbs of the capital, Damascus.
Human Rights Watch said those allegations against the opposition lack credibility and do not match the evidence it has seen.
The group's emergencies director, Peter Bouckaert, said the attacks should "refocus the international debate" on chemical weapons deterrence and also protecting Syrian civilians.
More than 100,000 people have been killed since the Syrian conflict began in 2011.