The United States said on Tuesday it was evaluating allegations of chemical weapons use in Syria, but dismissed charges that the opposition had used such weapons in the two-year-old conflict.
The Syrian government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo.
"We are looking carefully at allegations of ... chemical weapons use, we are evaluating them," White House spokesman Jay Carney told reporters. "We have no evidence to substantiate the charge that the opposition has used chemical weapons," he said.
"We are deeply skeptical of a regime that has lost all credibility and we would also warn the regime against making these kinds of charges as any kind of pretext or cover for its use of chemical weapons."
Two leading U.S. lawmakers, while cautioning that reports of chemical weapons use by the Syrian government had not been confirmed, expressed concern after being briefed on the situation by Obama administration officials.
"It is serious, and it may well take some action," Democratic Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, said after the closed-door afternoon briefing. She spoke in an interview with CNN.
"I think the White House needs to complete an assessment and make some statement as to what action the United States will take," she said, according to a preliminary transcript of the broadcast obtained by Reuters.
Republican Representative Mike Rogers, who chairs the House Intelligence Committee, told CNN: "Do I believe that they have configured weapons and may have used them? Yes. However, we don't know for sure, and for certain. I think that will happen within hours, if not days."
The Pentagon said it was monitoring the situation.
"I have no information at this time to corroborate any claims that chemical weapons have been used in Syria," Pentagon spokesman George Little said. "The use of chemical weapons in Syria would be deplorable."
Carney reiterated that President Barack Obama has said there would be consequences and the government of President Bashar al-Assad would be held accountable if chemical weapons were used. Carney would not say what those consequences would entail.
The United States has been concerned that the Assad government would consider using chemical weapons as it becomes "increasingly beleaguered and finds its escalation of violence through conventional means inadequate," Carney said. "This is a serious concern."
He said the U.S. position was still that it was supplying only non-lethal aid to the Syrian opposition. "Our position is and remains that we are not supplying lethal assistance to the opposition," Carney said.
(Editing by Mohammad Zargham and Peter Cooney)