A senior U.S. official told Fox News Wednesday that Syrian forces had loaded bombs with components of sarin gas, a deadly nerve gas. They have 60 days to use these bombs until the chemical mixture expires and has to be destroyed.

NBC News which first reported this latest escalation in the Syrian civil war, cited sources saying bombs filled with a sarin component have not yet been loaded onto planes, but the Syrian military is prepared to use these chemical weapons against civilians pending orders from Assad.

The sarin could be delivered in several ways but is believed to have been placed in fracturable canisters that can be dropped from planes, according to a senior US military source.

"We think they have it in aerosol form," the source told Fox News.

The United States has said chemical weapons use would be unacceptable and would trigger greater Western intervention in the conflict.