The 71-year-old told fans in a blog post that he hopes The Winds of Winter will be finished "next year".
He wrote: "I have to confess, after half a year of pandemic, quarantine, and social distancing, I am showing signs of cabin fever… half of which is quite literal in my case.
"Yes, I am in an actual cabin in the mountains... if nothing else, the enforced isolation has helped me write.
"I am spending long hours every day on THE WINDS OF WINTER, and making steady progress."
The writer said Game of Thrones fans should not expect the book to be finished "tomorrow" or "published next week", saying he still had "a long way to go".
Martin's bestselling series is set in the fantasy kingdom of Westeros, and was turned into a hugely popular TV show by HBO.