Looking for a late 70's/early 80's action movie.

It was about some people on an island. Two groups fighting eachother. They were throwing plastic jugs with gasoline (with a match in the lid) at eachother. In one of the last scenes, some people have taken defensive position inside a shack/little house (maybe with guns) , and the other guys manage to get them out of there by pouring gasoline in a chute that leads inside the house and lighting it. The ones inside comes out horribly burned. Saw it like 38 years ago.