The unnamed Obama administration official released a statement describing the attack Wednesday on rebels in the Damascus area as "indiscriminate," The New York Times reported.
"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians," the official said.
The official also suggested President Obama is likely to respond with military force.
The United Nations said its chemical weapons inspectors would visit the site of the alleged attack in Damascus suburb Monday. But the official said the inspection could be meaningless because the area has been "corrupted" by government shelling.
Syria agreed Sunday to allow the U.N. inspectors into the area where rebels claim government troops deployed chemical weapons last week, causing several hundred deaths. The U.N. team would be the first group of outsiders to visit the scene and independently verify the rebel claims.
The government this weekend acknowledged there had been a chemical incident, but blamed the rebels for unleashing the gas, CNN said.
The U.N. said in a written statement the Syrian government intended to observe a cease-fire while the U.N. team was doing its work and urged the rebels to hold their fire as well. "The secretary-general would like to reiterate that all relevant parties equally share the responsibility of cooperating in urgently generating a safe environment for the mission to do its job efficiently and providing all necessary information," it said.
Syria's SANA news agency said President Bashar Assad's regime was anxious to work with the inspectors "to expose the false allegations of the terrorist groups accusing the Syrian forces of using chemical weapons."
The United States, Russia and other nations have called for independent evidence to clarify what actually happened last week. U.S. Secretary of State John Kerry pressed that wish and the "gravity of the situation" in a series of telephone calls Saturday to his counterparts in Saudi Arabia, Turkey, the Arab League and other regional players, a State Department source told the Times.
Meanwhile, the chemical incident continued to raise tensions between the United States and Iran. The U.S. Navy had four destroyers off Syria Sunday, each capable of deploying cruise missiles capable of reaching specific targets deep inside Syria.
Syria's decision to allow the U.N. team into the scene came after a rare telephone conversation between Kerry and Syria's foreign minister. The Wall Street Journal said some analysts suspected Kerry delivered an ultimatum to the government that included the threat of military intervention.
President Obama had previously declared the use of poison gas by the Assad regime to be a "red line" and warned Syria it would face consequences from the international community.
Iran, however, this weekend warned the United States was getting close to another "red line." The deputy chief of staff of the Iranian military told the semiofficial Fars new agency there would be "severe consequences" for any U.S. move against its Syrian allies.