Today, Prime Minister David Cameron will chair a meeting with military chiefs who, No 10 confirmed, have already drawn up contingency plans. Meantime, the US Secretary of State, Chuck Hagel, has made clear that America is "ready to go" if the Commander-in-Chief Barack Obama gives the green light.
The US Navy's Sixth Fleet has four Tomahawk Cruise missile-carrying destroyers on stand-by in the eastern Mediterranean along with, reports suggest, a missile-firing submarine.
In the Persian Gulf off Bahrain, there is the USS Truman carrier battle group that includes the guided-missile cruiser USS Gettysburg. The USS Nimitz aircraft carrier with its 70 strike fighters is sailing in the same stretch of water.
The Royal Navy is thought to have HMS Tireless, a Plymouth-based submarine armed with Cruise missiles, in the Mediterranean. It was reported to be in Gibraltar over the weekend.
RAF Akrotiri, on Cyprus, could also be considered as a potential launchpad for air strikes, as could land bases in Turkey.
French air power could also play a part in any military strike.
Given that Russia has supplied Syria with state-of-the-art air defence systems, it is thought the involvement of allied aircraft is unlikely.
Instead, US administration and defence officials have said the most likely military move would be the launch of Tomahawk missiles off ships in the Mediterranean.