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.