The White House said Thursday it's believed between 100 and 150 people died from chemical weapon attacks in Syria, according to multiple, independent streams of information.
Sarin was used multiple times by the Assad regime, according to the White House.
President Barack Obama previously said the use of chemical weapons was a "red line" that would trigger greater U.S. involvement in the conflict.
U.S. intelligence had earlier indicated chemical weapons had been used in Syria but it was not clear by whom. Assad has been battling opposition forces for more than two years in the country resulting in 93,000 dead.
The earlier intelligence assessments have been corroborated, according to the Obama administration. It was not immediately announced what response the administration would offer.
Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
Officials said Obama was considering both political and military options, but suggested deeper involvement may not be imminent.
"We've prepared for many contingencies in Syria," said Ben Rhodes, Obama's deputy national security adviser. "We are going to make decisions on further actions on our own timeline."
There was no immediate decision to approve military operations in Syria.
Congress was also being notified of the chemical weapons determination on Thursday in classified documents sent to Capitol Hill, the officials said. The finding was aided by evidence sent to the United States by France, which along with Britain, announced last week that they had determined that Assad's government had used chemical weapons in the two-year conflict.
The Associated Press contributed to this report.