“More review is needed,” the senior U.S. official told Reuters, speaking on condition of anonymity.

The disclosure came as the U.S. Director of National Intelligence James Clapper said at a Senate hearing that the Syrian regime of President Bashar al-Assad “appears quite willing to use chemical weapons against its own people.”

“We receive many claims of chemical warfare use in Syria each day and we take them all seriously, and we do all we can to investigate them,” Clapper said.

Still, Clapper declined to say in the public hearing what the U.S. intelligence community had concluded about allegations of chemical weapons use by the Assad regime. He said keeping that assessment private was important “to protect the fragile, critical intelligence we need to assess the situation.”