British troops could be deployed to intervene in Syria in the event of a major humanitarian crisis, the head of the armed forces says.

General Sir David Richards, the Chief of the Defence Staff, said contingency plans were being drawn up in case the onset of winter saw a worsening of conditions on the ground.

He indicated that any intervention would be limited and would need the support of people inside Syria in the areas where assistance was being provided.

Nevertheless such a move 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 Assad.

"It would be a huge effort. We would be very cautious about it," he told BBC1's The Andrew Marr Show.

"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."

He said he expected the humanitarian situation to deteriorate over the winter which could lead to calls to intervene "in a limited way".

"Obviously we develop contingency plans to look at all these things. It is my job to make sure that these options are continually brushed over to make sure that we can deliver them and they are credible," he said.

"The main thing for now that we are all focusing on is to contain the crisis so that it doesn't spill over into countries like Lebanon, Jordan, Turkey.

"That's our primary focus but that would also accommodate a humanitarian crisis because we could help deal with that through that primary mechanism. It is certainly something we have got to look at.