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: