It is not just the Assad regime that is bracing itself. Jihadists fear the US may seek to kill two birds with one stone by targeting them as well, amid fears that they emerge the winners if the Assad regime is overthrown. On Foreign Policy, Charles Lister writes:
In a note entitled "Important Instructions... Before the US initiates its Mission," distributed via social media on 27 August, senior Fatah al-Islam leader Abdullah Shaker (Abu Bakr) claimed: "For each and every missile that strikes a missile site, there will be another that targets the mujahideen's positions," suggesting such strikes would aim to kill as many jihadist leaders as possible. Shaker went on to advise all jihadists to "change your positions, take shelter, and do not move in public," and underlined how previous experiences in Mali, Iraq, and Afghanistan had seen "the mujahideen destroyed in a very short time," as the necessary precautions were not undertaken. Shaker also advised against any attempts to deploy anti-aircraft weapons against US "raids" as this would "practically be suicidal."
Similar notes of warning and advice have been distributed by known Jabhat al-Nusra and ISIS members and sympathizers, including an anonymous "brother familiar with the American media." He suggested on 25 August that in addition to US-led strikes targeting Syrian "radar systems, air defence systems, the chemical weapons industry, and stocks of Scud missiles," a second set of strikes would target "the training camps of Jabhat al-Nusra and ISIS, the group's top tier leaders, and the sharia courts."
http://www.theguardian.com/world/middle-east-live/2013/aug/30/syria-us-military-britain-live
It may just be paranoia in the context of the Syrian situation, but I suppose their fear is based on past experience elsewhere.