Washington (AFP) - The self-proclaimed mastermind of the September 11 attacks, Khalid Sheikh Mohammed, has released a manifesto claiming that the Koran forbids the use of violence to spread Islam.

The document, published Tuesday by The Huffington Post and Britain's Channel 4 News, marks Mohammed's first public communication since 2009, when the US government officially accused him of terrorism.

Mohammed, the most high-profile of the five men accused over the 2001 attacks that killed nearly 3,000 people on US soil, has been held at the US detention facility in Guantanamo Bay, Cuba since 2006.

In a major departure from his previous position, Mohammed said that "the Holy Koran forbids us to use force as a means of converting!"

He also tried in the 36-page document to convince his American captors, prosecutors, lawyers and members of his military tribunal to convert to Islam.

"It is my religious duty in dealing with any non-Muslims such as the people in the court (the judge, the prosecution, attorneys, etc.) to invite them to embrace Islam," Mohammed wrote.

"I realize very well that you have heard about Islam and know much about it. But it is my own belief that Allah will ask me on the Day of Judgment why I did not invite these people to Islam?"

Mohammed said he was "very happy" in his cell, adding: "My spirit is free even while my body is being held captive."

Mohammed said he has been "neither sad nor distressed" in his confinement "because I have been with the Only One True God."

The document was declassified last month by military judge James Pohl.