General Sir David Richards said plans were being drawn up in case winter made conditions on the ground worse.
Any intervention would be ‘limited’ and needed the support of people inside Syria, he said.
But it would be seen as a potential step towards a full-scale military intervention bringing British forces directly into conflict with the regime of president Bashar al-Assad.
‘It would be a huge effort. We would be very cautious about it,’ the chief of the defence staff told the BBC.
‘There is no ultimately military reason why one shouldn’t and I know that all these options are quite rightly being examined. But we are some way off.’
The announcement came as Israeli forces fired ‘warning shots’ into Syria after a stray mortar exploded near a Jewish settlement in the occupied Golan Heights.
Israel said foreign fire ‘will not be tolerated’ and ‘responded to with severity’.
The two countries fought over the disputed territory in 1973.