More definitive proof was required, according to officials, to prompt further U.S. involvement in the conflict. President Obama has repeatedly said the use or transfer of chemical weapons by the Syrian government would cross a "red line."
The White House said the Syrian regime is believed to have used the chemical agent sarin on a small scale. Details of casualties were not clear.
The Syrian civil war has dragged on for more than two years, with an estimated 70,000 dead.
Sarin is an odorless nerve agent that can be used as a gas or a liquid, poisoning people when they breathe it, absorb it through their skin or eyes, or take it in through food or water. In large doses, sarin can cause convulsions, paralysis and death. According to the Centers for Disease Control and Prevention, people usually recover from small doses, which may cause confusion, drooling, excessive sweating, nausea and vomiting.
Obama has said the use of chemical weapons would be a "game-changer" in the U.S. position on intervening in the Syrian civil war, and the letter to Congress reiterated that the use or transfer of such weapons in Syria was a "red line for the United States." However, officials quickly made clear that a stepped up U.S. response was not imminent.
The White House emphasized a need for the completion of a stalled U.N. investigation.
But it's unclear whether U.N. inspectors will ever be able to conduct a full investigation in areas where there is the most evidence of chemical weapons use. The Syrian government has so far refused to allow the U.N. experts to go anywhere but Khan al-Assal, where Assad's government maintains the rebels used the deadly agents.
Last month, British and French ambassadors to the United Nations told Secretary General Ban Ki-moon that the government used chemical weapons near Aleppo, in Homs and possibly in the capital of Damascus. Pressure mounted on the U.S. this week when two key allies in the Middle East - Israel and Qatar - also said there was evidence that Assad had used chemical weapons.
The U.S. believes the use of chemical weapons "originated with the Assad regime." That is consistent with the Obama administration's assertion that the Syrian rebels do not have access to the country's stockpiles.
U.S. commanders have laid out a range of possible options for military involvement in Syria, but they have made it clear that any action would likely be either with NATO backing or with a coalition of nations similar to the NATO-led overthrow of Libyan dictator Col. Moammar Gadhafi in 2011.
The military options could include establishing a no-fly zone or a secured area within Syria, launching airstrikes by drones and fighter jets and sending in tens of thousands of ground forces to secure the regime's chemical weapons caches.
The Associated Press contributed to this report.
