The United States was Wednesday studying a Russian report that Syrian rebels used sarin gas in an attack on Aleppo, but believes the opposition does not have access to such weapons.
"I understand we just received the report this morning. We'll of course need time to review it," said State Department spokeswoman Jen Psaki.
Russia's envoy to the United Nations said Tuesday that experts from his country had been to the scene of an attack at Khan al-Assal near the northern city of Aleppo in March and gathered firsthand evidence.
"The United States continues to have no reliable corroborated reporting to indicate that the opposition in Syria has used chemical weapons," Psaki insisted.
She also accused Russia of "blocking an effort by the Security Council to allow UN access into Syria to investigate any and all credible allegations."
Last month the administration of President Barack Obama finally acknowledged that the Syrian regime of President Bashar al-Assad had used chemical weapons, including sarin, in attacks which killed up to 150 people.
The admission prompted the administration to toughen its stand and vow to provide more direct military aid to the opposition "different in both scope and scale" to what it has so far provided to the rebels.
However, it remains unclear if any of that new aid has yet been delivered as the rebels battle to oust Assad, in a conflict which has claimed 100,000 lives.
One senior Arab official, who asked to remain anonymous, said recently that no such increase in military aid had so far been detected by his government.