The White House said on Tuesday it was looking carefully at allegations that chemical weapons were used in Syria, but said it had no evidence to substantiate charges that the opposition had used such weapons.
"We are looking carefully at the information as it comes in," White House spokesman Jay Carney told reporters. "This is an issue that has been made very clear by the president to be of great concern to us."
The Syrian government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo on Tuesday in what would, if confirmed, be the first use of such weapons in the two-year-old conflict.
The United States has been concerned that Syrian President Bashar al-Assad would consider the use of chemical weapons as his grip on power becomes "increasingly beleaguered and finds its escalation of violence through conventional means inadequate," Carney said. "This is a serious concern."
President Barack Obama has been clear that if Assad and his forces used chemical weapons there would be consequences, Carney said, but he did not detail what those consequences would be.
He said the U.S. position is still that it will supply 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.