HBO's most-watched series, the medieval fantasy drama "Game of Thrones," will likely continue through three more seasons, a network executive said [last week,] as he defended the show's gritty violence.The sixth season is currently being filmed.Executives have planned to run at least seven seasons and are discussing with the show's creators how much longer the show will extend beyond that, said Michael Lombardo, president of HBO programming. "They are feeling two more years after six," Lombardo said at a Television Critics Association meeting. Lombardo said he'd like to have more beyond that and would also be open to a prequel to the story of the epic battle for the Iron Throne in the fictional kingdom of Westeros, based on George R.R. Martin's "A Song of Ice and Fire" books.This year, "Game of Thrones" hit 20 million viewers on average for its fifth season, which received 24 Emmy nominations including a best drama nod.It also generated controversy for a brutal rape scene and burning of a child. Lombardo defended its graphic depictions as part of the story and said he supported show creators Dan Weiss and David Benioff."This show has had violence as part of its many threads from first episode," he said. "There are no two showrunners more careful about not overstepping what they think the line is."The fifth season also ended by killing off one of its lead characters and fan favorite, Jon Snow, but viewers have been speculating that the character will return - not that Lombardo was giving away anything on Thursday. "Everything I've seen, heard and read, Jon Snow is dead," he said. "Game of Thrones" this year also helped launch HBO's standalone streaming service HBO Now, where viewers can watch HBO programming at any time without a cable subscription, as television continues to move toward on-demand viewing.Lombardo told Reuters he was happy with HBO Now's rollout but would not reveal subscriber numbers. He also said marketing has become more challenging amid an explosion of critically acclaimed shows. Viewers of HBO, a unit of Time Warner Inc, routinely tune in Sunday nights, but other days are less certain, he said. "I worry about that all the time," he said. "Particularly with a digital platform and on-demand where things are available, I think quality will ultimately win out."