The United States today rejected Russian claims raised at the United Nations that Syrian rebels had used chemical weapons.
"We have yet to see any evidence that backs up the assertion that anybody besides the Syrian government has the ability to use chemical weapons, (or) has used chemical weapons," White House spokesman Jay Carney said.
Russia earlier handed over evidence to the United Nations which its envoy said indicated Syrian rebels used sarin gas in an attack in March.
Ambassador Vitaly Churkin said Russian experts had been to the scene of the attack at Khan al-Assal near Aleppo and gathered firsthand evidence.
The Russian action risks reigniting an international dispute over the use of chemical weapons in the 26-month-old war, in which the United Nations says up to 100,000 people have been killed.
The Syrian government has refused to let a UN inspection team into the country, but this week invited UN officials for talks on the investigation.
Carney again called on Syrian President Bashar al-Assad to allow foreign investigators to probe claims of chemical weapons use.
"The way to answer this question is to allow the United Nations to investigate," he said.
"Our ability as an international community to investigate the use of chemical weapons in Syria is hampered by Assad's refusal to allow a United Nations investigation."
Britain, France and the United States say they have handed over evidence to UN experts indicating that Assad's forces have used chemical weapons in the conflict.
