Britain's military chiefs have drawn up contingency plans to provide Syrian rebels with maritime, and possibly air, power in response to a request from David Cameron, senior defence sources said on Monday night.
However, they said the UK would act only if the US did so and made it clear that British chiefs of staffs are seriously worried about the consequences of intervening in the Syrian crisis.
Military sources have privately been expressing growing concern about pressure from Downing Street.
Officials compared it to the pressure applied by Cameron before Britain and France took the lead in the campaign of air strikes on Libya early last year.
Options have been sent by defence chiefs to No 10, sources told the Guardian on Monday night. However, one source added: "We are a long way from doing anything." An official said: "The US is leading the way. We are not there yet."
At Cameron's request, General Sir David Richards, chief of the defence staff, chaired a meeting in London a month ago, sources said. In attendance were senior military figures from France, the US, Turkey, Jordan and the Gulf states of Qatar and the UAE who, with Saudi Arabia, have been in the vanguard of those supporting the Syrian rebels.
Most British military chiefs say intervening in Syria would be very different from last year's Libyan operation because of the sheer number of ground-to-air missiles in the hands of Syrian forces.
Other military sources say the capabilities of the Syrian regime have been exaggerated and most of its weaponry is old.
The British backed a Nato-implemented no-fly zone in Libya to protect rebels against the regime of Muammar Gaddafi, but this was heavily dependent on the US to supply the drone and aircraft support to make that possible. Britain may yet be willing to back a no-fly zone to protect Syrian refugees.
William Hague, the foreign secretary, has said the more the opposition groups unify around a coherent programme built on respect for human rights and ethnic tolerance, the more the British government can provide aid.