What complicates any U.S. military support for the opposition is that many of the rebel fighters are militants with pro-al Qaeda sympathies, the same stripe of militants America has battled in Iraq and Afghanistan.
They include an group called the al-Nusra Front, a rebel group that the United States says has links to al Qaeda.
Earlier this year, the United States said its intelligence analysts had concluded "with varying degrees of confidence" that chemical weapons had been used in the Syrian civil war. But Obama said then that "intelligence assessments alone are not sufficient."
As recently as last week, France's foreign minister said sarin gas had been used several times in the Syrian civil war, citing results from test samples in France's possession.
In early May, the head of the U.N. Independent International Commission of Inquiry on Syria said that evidence points to the use of sarin by Syrian rebel forces. But the commission later issued a news release saying it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
In April, the head of the Israeli military's intelligence research said the Syrian government is using chemical weapons against rebel forces.
Sarin gas can be hard to detect because it is colorless, odorless and tasteless. It can cause severe injuries, including blurred vision, convulsions, paralysis and death, to those exposed to it.
Analysts believe the Syrian government may have one of the largest stockpiles of chemical weapons in the world. Specifically, the supply is believed to include sarin, mustard and VX gases, which are banned under international law. Syria has denied the allegation.
The Chemical Weapons Convention prohibits the production, stockpiling and use of chemical and biological weapons. Syria is not one of the 188 signatories to the convention.
In recent months, reports have repeatedly surfaced that Syrian forces have moved some of the chemical weapons inventories possibly because of deteriorating security in the country, raising fears the stockpile could fall into the hands of al Qaeda-linked groups working with the opposition should al-Assad's government fall.
As a result, the United States has been talking with neighboring countries about the steps needed to secure the weapons should al-Assad be forced from office.
The U.N. estimates that more than 92,000 people have been killed since the conflict began in March 2011, when a brutal government crackdown on Arab Spring-inspired protests devolved into an armed conflict.