The film received its premiere in Tokyo last month

The film topped the US National Board of Review (NBR) list for best film.

Helen Mirren was named best actress for The Queen, while Forest Whitaker won best actor for his portrayal of Idi Amin in The Last King of Scotland.

NBR voters include teachers and historians as well as those who work in the film industry. The awards season culminates in February with the Oscars.

Martin Scorsese was named best director for crime thriller The Departed, which also won the award for best ensemble cast.

Djimon Hounsou won best supporting actor for Blood Diamond, about illegally traded jewels.

Pedro Almodovar's Volver won best foreign film

Catherine O'Hara was named best supporting actress for her role in spoof comedy movie For Your Consideration.

Other winners include Disney for Cars in the best animation category and Zach Helm took home the best original screenplay prize for Stranger Than Fiction.

Eastwood's Letters from Iwo Jima tells the story of the battle for control of the Japanese island during World War II, told from the perspective of Japanese soldiers.

"Letters from Iwo Jima is probably Mr Eastwood's masterpiece and perhaps one of the greatest films of our time," said NBR president Annie Schulhof in a statement.

The film, due to be released in the US on 20 December, follows Eastwood's earlier movie on the same subject, Flags of our Fathers, which was told through the eyes of the Americans.

Letters from Iwo Jima topped the NBR's list of 10, which also included Babel, Blood Diamond, The Departed, The Devil Wears Prada, Flags of Our Fathers, The History Boys, Little Miss Sunshine, Notes On A Scandal, and The Painted Veil.