The United States said Tuesday it had seen "no evidence" that Syrian rebels have used chemical weapons and warned it would be "totally unacceptable" for the regime in Damascus to use such arms.
Responding to claims by the Syrian government and its ally Russia, White House spokesman Jay Carney said the United States has "no evidence to substantiate the charge that the opposition has used chemical weapons."
The Syrian government claimed 25 people were killed in a chemical weapons attack by rebels in Aleppo, which the opposition denied, saying in turn that the regime might have used the banned weapons.
State Department spokeswoman Victoria Nuland said US ambassador to Moscow, Michael McFaul, would also be seeking clarification from Russian authorities.
"If you read their statement, they appear to have taken the regime's word for it," she said, after the Russian foreign ministry backed Damascus's claim.
"We would be deeply skeptical, as we have said, about accepting the word of a regime that's lost all credibility."
She dismissed the reports as nothing "more than the regime's continued attempts to discredit the legitimate opposition and distract from its own atrocities committed against the Syrian people."
Washington was meanwhile looking "carefully at allegations made by the opposition" that it was in fact the Syrian regime which had used chemical weapons in the attack.
"We will consult closely with our partners on this," Nuland said.
Carney reiterated President Barack Obama's longstanding warning that if the regime of President Bashar al-Assad used chemical weapons "there will be consequences and they will be held accountable."
"It is important that as fighting in Syria intensifies and the regime becomes more desperate, that the United States and the international community make absolutely clear to Assad that the use of chemical weapons would be totally unacceptable," he said.
Pentagon spokesman George Little said that any use of chemical weapons in Syria "would be deplorable" and reiterated that "the Syrian regime has an absolute obligation to maintain positive control and security of its stockpiles."