Syria has long maintained that rebels, not government forces, are behind the use of chemical weapons. It also went to the United Nations with its claims, but al-Assad would not allow U.N. inspectors into the country to try to verify the claims.
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.
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
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.
U.S. officials have been closely monitoring Syria's chemical weapons stockpiles and are certain they remain in the control of al-Assad's regime, Rhodes said, adding that it would be too dangerous to destroy the chemical weapons stockpiles from afar.
As recently as last week, the French 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."