The assessment also suggested that a nerve agent was used in the suspected poison attack, but further evidence was needed to determine the type of agent, US government sources told the news agency.
President Donald Trump has threatened to respond "forcefully" to the alleged chemical attack despite strong warnings from Russia.
After meeting his cabinet and top generals on Monday, Trump told reporters "we have a lot of options militarily and we'll be letting you know pretty soon... probably after the fact."
In a pair of tweets on Sunday, President Donald Trump appeared to blame the Syrian government and Russia for Saturday’s chemical incident in the suburb of Damascus, and warned of “a big price.”
Trump also called out Russian President Vladimir Putin for his role in Syria’s conflict that just entered its eighth year.
The US president and his French counterpart Emmanuel Macron agreed in a phone call that chemical weapons had been used in Douma.
“Both leaders strongly condemned the horrific chemical weapons attacks in Syria and agreed that the Assad regime must be held accountable for its continued human rights abuses,” a White House statement said.
Trump’s tough language has raised concern that he might order a military attack against Syrian government positions a year after he launched a barrage of missile strikes against a Syrian air base following allegations of a chemical attack.
The US president said Monday he would make a decision on a response, probably by the end of the day.
Speaking at a Cabinet meeting, Trump said he was in touch with military leaders and advisers to determine who was responsible for the gas attack.
Meanwhile, the State Department said Monday it was consulting with US allies on a possible response, stressing that "there will be consequences for this unacceptable atrocity."
Defense Secretary James Mattis told reporters in Washington that he would not rule out military action such as air strikes if it was determined that the Syrian government was responsible.
Mattis accused Russia of falling short on its obligations to ensure that Syria abandoned its chemical weapons capabilities.
"The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons," he said.
Damascus strongly rejected the allegation of using chemical munitions and said the so-called Jaish al-Islam terrorist group, which has dominant presence in Douma, was spreading the accusations “in a blatant attempt to hinder the Army’s advance.”