The stars of the hit US sitcom The Big Bang Theory have yet to ink new contracts and are believed to be holding out for $US1 million ($1.07m) an episode.

US network CBS has already green lit three more seasons of The Big Bang Theory, which airs in Australia on the Nine Network, but negotiations with actors has hit a snag.

Big Buck Theory ... Jim Parsons (who plays Sheldon Cooper), Johnny Galecki (Leonard Hofstadter), Kaley Cuoco (Penny Kaley), Simon Helberg (Howard Wolowitz) and Kunal Nayyar (Raj Koothrappal) are holding out for a better pay deal.

The Hollywood Reporter says actors Jim Parsons (who plays Sheldon Cooper), Johnny Galecki (Leonard Hofstadter), Kaley Cuoco (Penny Kaley), Simon Helberg (Howard Wolowitz) and Kunal Nayyar (Raj Koothrappal) have not signed and are working together to extract a huge pay increase.

The US entertainment website quotes industry sources and says there has been no movement in the negotiations which started in September.