I made a head, marked some seams and then unwrapped it.
Then I made one more seam. Is it possible to straightforwardly select a seam edged area in 3D Edit Mode to unwrap that new area only?
Its only what you have selected in the 3d view that will be unwrapped
Yes but how to easily select the area that is surrounded with seam? Currently I select face by face but that takes time.
If you want to select uv islands in the 3d view either hovee your curser over an island and press L or select a face on one island and Ctrl+L to select linked faces. This only works when in face select mode.