The show is currently filming its fifth season, which is due to air in 2015.
While a seventh run is yet to be confirmed, its phenomenal ratings success makes it a strong possibility.
The season four finale of Game of Thrones attracted 7.1 million viewers, helping the show topple The Sopranos to become HBO's most-watched series ever.
The new contract splits the stars into three tiers.
The "A-tier", which pays the US$300,000, includes Peter Dinklage (Tyrion Lannister), Kit Harington (Jon Snow), Lena Headey (Cersei Lannister), Emilia Clarke (Daenerys Targaryen) and Nikolaj Coster-Waldau (Jaime Lannister)
Natalie Dormer (Margaery Tyrell), Maisie Williams (Arya Stark) and Sophie Turner (Sansa Stark) fall under the "B-tier", while a still-lower tier of regular actors will reportedly receive much smaller pay increases.
While the news appears at first to provide one massive spoiler after another, the new contracts do not reportedly guarantee the actors who have signed will live to see a seventh season.
Martin is notorious for unexpected gruesome deaths and he still hasn't finished his books.
With more than 20 characters having already found their fate in the first four series, anything is possible.
Especially when winter is coming.
- AAP