A statement issued by Ben Rhodes, deputy national security adviser for strategic communications, said 100-150 people were killed by the weapons, including the nerve agent sarin.
"Following a deliberative review, our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," the statement said.
President Obama has said the use of chemical weapons by the Assad regime was a "red line" that would trigger U.S. reaction but there was no immediate indication what specific actions the United States would take.
The assessment followed word from the United Nations that violence in Syria has claimed 93,000 lives since demonstrations began in March 2011 to overthrow President Bashar Assad.
"While the lethality of these attacks make up only a small portion of the catastrophic loss of life in Syria, which now stands at more than 90,000 deaths, the use of chemical weapons violates international norms and crosses clear red lines that have existed within the international community for decades," Rhodes' statement said. "We believe that the Assad regime maintains control of these weapons. We have no reliable, corroborated reporting to indicate that the opposition in Syria has acquired or used chemical weapons."
Assad has accused the rebels of unleashing the chemical weapons.
The statement chastised Syria for preventing international investigators from checking reports of alleged chemical weapons use.
"The president has been clear that the use of chemical weapons -- or the transfer of chemical weapons to terrorist groups -- is a red line for the United States, as there has long been an established norm within the international community against the use of chemical weapons. Our intelligence community now has a high confidence assessment that chemical weapons have been used on a small scale by the Assad regime in Syria. The president has said that the use of chemical weapons would change his calculus, and it has," the statement said.
The statement said assistance to the Supreme Military Council, the main rebel group -- has been "augmented" and will be augmented further in the wake of the assessment.