Game of Thrones author admits HBO series is catching up fast with his novels but 'it's almost out of my hands'

George RR Martin has admitted the Game of Thrones TV adaptations of his novels are catching up fast but with two books still to come there is little he can do.

There have been four series of Game of Thrones based on the first four novels of his series A Song of Ice and Fire.

The next series will be based on a published book and the following two series will, in theory, have to wait until he completes the remaining two books.

On Tuesday he told the Edinburgh international book festival: "They've certainly caught up to me, They are writing 60-page screenplays and I'm writing 1,300-page books.

"There is no doubt the show is moving very, very fast. But whether I'm concerned or not concerned makes no difference. I can only write one word at a time, one sentence at a time, one book at a time.

"I'm writing the books as fast as I can write the books and the show is moving along – we'll see what happens, but it's almost out of my hands so I can't worry about it. I just have to worry about telling the stories as best I can."

After talking about his books on Monday evening Martin returned to the festival for a Guardian event in which he spoke about the hugely popular HBO series.

He disagreed with one questioner who said there was more sex on the TV screen than in the books, but admitted he had not counted. "Generally I'm in favour of sex and I think a large portion of the audience is too."

But he said the reaction to sex scenes, particularly in the US, was strange.

"It has always astonished me that there is so much more controversy about the sex than about the violence. That says something about us.

"I can write a scene describing in detail a penis entering a vagina and there will be a portion of the audience who will get very upset by that and then I can write a scene about an axe entering a human skull, nobody will blink."

Martin does screenwrite one episode of each series but he concentrates very much on the novels, he told the festival.

The vast fantasy books have a dizzying array of characters and Martin admitted he had been caught out and made mistakes, changing the sex of a horse once, and "I'm terrible with the eye colours." The mistakes were always spotted by fans, he said.

In a wide-ranging interview Martin said that growing up in blue-collar New Jersey, he wanted to be a spaceman when he grew up.

Or a superhero. He particularly was attracted to the Green Lantern because all he had to do was find a magic ring, whereas Batman "seemed to involve a lot of exercise", he could not be Superman because he was not from Krypton and the Flash was hit by painful lightning.

After college Martin began writing novels and in the 1980s he became a scriptwriter on shows including The Twilight Zone and Beauty and the Beast.

He wrote a lot of scripts that were never made but it improved him as a writer, he said. "My years in Hollywood sharpened my dialogue and my sense of structure."

Martin began writing A Song of Ice and Fire in 1991 and admitted he put so much in the books, so many battles and characters, that he always thought they were unfilmable.

Nevertheless, the movie world came knocking after the success of Peter Jackson's Lord of the Rings series. Martin told his audience: "Every studio was saying: 'We've got to get ourselves one of these, are there more of these fantasy thingies out there?'"

Martin turned them down but he said it got him wondering if they could they be done on television.

"It could not be done for traditional television, like CBS, because there's too much sex and violence in them and I didn't want to lose the sex and violence, I didn't want to tone it down and produce a tepid version."

But HBO's proposal won him over and he is more than pleased with the results. "It is one of the most faithful adaptations I've ever seen."