The United States says it is doesn't believe the rebels in Syria have used chemical weapons.
White House spokesman Jay Carney told reporters that it was more probable that government forces had used such weapons.
"We are highly skeptical of any suggestions or accusations that the opposition used chemical weapons," he said. "We find it highly likely that chemical weapons, if they were in fact used in Syria - and there is certainly evidence that they were - that the Assad regime was responsible."
Carney was speaking on May 6 after an investigator with a UN inquiry said evidence gathered so far suggested the rebels, not the government of Bashar al-Assad, had used such weapons.
The Independent International Commission of Inquiry on Syria, however, distanced itself from the remarks made by Carla Del Ponte to Swiss TV on May 5, saying it had not reached any conclusions yet.
Del Ponte, one of the investigators on the commission, said interviews with victims and doctors treating them indicated the rebels had used sarin gas.
The rebels rejected the claim.
A Syrian opposition leader stressed that only the regime had such capabilities, and maintained that it would "continue gathering evidence" and hand it over to the inquiry commission.
"Since [Syrian President] Bashar al-Assad is not allowing [international investigators] to get [into Syria] that's a strong proof that he is the one using them," said Moulham al-Droubi, a member of the National Council's General Secretariat. "The Syrian intelligence apparatus can send people who claim to be within the opposition and make such statements and give false [testimony] that the opposition is using chemical weapons. So it is an intelligence game. "
Doctors Without Borders -- a medical charity that runs four hospitals in rebel-held areas of Syria -- said on May 6 that it has not come across patients suffering from the use of chemical weapons.
Assad's government and the rebels have accused each other of using chemical weapons.
U.S. President Barack Obama has said the use of such weapons by Damascus would be a 'game changer.'
With reporting by dpa and Reuters