BRUSSELS—The civilian head of the North Atlantic Treaty Organization gave an upbeat assessment of the progress of opposition forces in Libya, saying in an interview he expected rebel units in the west and east of the country to join together.

Speaking at NATO headquarters in Brussels, Anders Fogh Rasmussen said opposition forces had advanced in recent weeks. "I think that progress will continue because the opposition forces are now more experienced, better trained, better coordinated. So, I think we'll see east and west join...