Eliyahu,  I would suggest you drill a small diameter hole in the screw
head and then insert an "Easy-Out" to back the screw out.  If you're not
familiar with them, they look sort of like a tapered drill bit with the
threads reversed.  As you "un-screw" the Easy-Out it grabs the old screw
tighter and tighter.  Then just take hold of it with a pliers and pull.

Dave Hall