President Assad was warned he would be "held accountable" if his forces use chemical weapons against rebel fighters in Syria, The New York Times reported.
The warning, from the US and European officials, came after reports the Syria military was moving weapons over the weekend. In July President Obama warned that any evidence that Assad was moving weapons in a threatening way is a "red line" that could prompt American intervention.
The Pentagon estimates that intervention to neutralise chemical weapons would require upward of 75,000 troops, according to The New York Times. An US official told the newspaper:
The president has made it clear that the use of chemical weapons in Syria would cross a red line for the United States. The Assad regime must know that the world is watching, and that they will be held accountable by the United States and the international community if they use chemical weapons or fail to meet their obligations to secure them.
General Richard Myers has told ITV News that the real threat in Syria is from Islamist groups using chemical weapons - not President Assad.
NATO agrees to deploys Patriot missiles along the Syria/Turkey border but denies claims that it is taking an active role in the conflict.