The Pentagon Thursday threatened legal action against a former Navy SEAL who published his account of the Osama bin Laden raid for breach of non-disclosure agreements.

In a letter obtained by Reuters, and subsequently released by the Pentagon, the DOD's top lawyer also warned that it was also considering legal steps against anyone "acting in concert" with the author of "No Easy Day."

The letter, addressed to "Mark Owen," the pseudonym under which the book was written, and sent via the attorney at Penguin Putnam publishing, identified two separate non-disclosure agreements he signed with the Navy in 2007 that legally committed him never to divulge classified information.

The SEAL also signed a "Sensitive Compartmented Information Debriefing Memorandum" when he left the Navy in April 2012, according to the letter. 

"You are in material breach and violation of the non-disclosure agreements you signed," wrote Jeh Johnson, the Pentagon's general counsel. "This commitment remains in force even after you left the active duty Navy."  

"The Department of Defense is considering pursuing against you, and all those acting in concert with you, all remedies legally available to us in light of this situation."

"We've got to get serious about leaks of classified information," a senior Pentagon official told NBC News. "As unpalatable as it may seem to go after this Navy SEAL, if we do nothing there is no deterrence, nothing to prevent others from doing the same."

Officials said last week they were surprised by the book, which was not vetted to ensure that no secrets were revealed.

The book has received widespread media coverage and the Pentagon letter noted that some copies have already been released, even ahead of the book's formal release next week.

"Further public dissemination of your book will aggravate your breach and violation of your agreements," the letter warned.

Earlier this week, the author said in a statement from his publisher that the book was written "with respect for my fellow service members while adhering to my strict desire not to disclose confidential or sensitive information that would compromise national security in any way."

But many in the special operations community have privately expressed disappointment in recent days over the book and the publicity it has received.

The author now faces threats against his life. An official al Qaeda website last week posted a photograph and the real name of the former Navy commando, calling him "the dog who murdered the martyr Sheikh Osama bin Laden."