HBO executives said on Thursday they were delighted and surprised at the success of new fantasy show "Game of Thrones" but they gave no guarantees that the TV series would cover all the books in the series.

"I don't know where the show will end, as opposed to the books," HBO programing chief Michael Lombardo told TV reporters.

"It would be fantastic to be able to say this show will go on for 10 years, and do every aspect of the books. I don't know that will be the case," he added. "We'll see, but we are doing this without any predetermined idea of the number of seasons."

"Game of Thrones" is currently reaching about 13.5 million viewers, including repeat episodes, HBO said. The series is derived from the five best-selling novels by George R.R. Martin published so far and production on season two is already underway.

"I think the show was a great surprise to a lot of people. The numbers bear that out. We couldn't be more delighted about the new season," said HBO co-president Richard Plepler.

But the two executives said decisions on the number of seasons would also depend partly on how long the writers and producers of the TV version want to remain involved.

Lombardo said however that the premium cable channel takes very seriously the passions of fans for the book series.

"That is a relationship that we are very aware and respectful of," he added.

"Game of Thrones" earned 13 nominations for the 2011 Emmy Awards, including best drama.

Game of Thrones HBO