The report, based on publicly available information, said North Korea renewed last year the on-again, off-again effort, first begun eight years ago and now apparently focused on elevating 26-year-old Kim Jong Un, the youngest of Kim Jong Il's three sons. The succession push is meant to coincide with the 2012 birth centennial of Kim Jong Un's grandfather, the country's revered founder Kim Il Sung.

The report was compiled by the U.S. Director of National Intelligence's Open Source Center. It is dated May 6, 2009.

"The recent signals have been extremely subtle, suggesting that they are designed to inform internal audiences without alerting outsiders," the report said.

The Open Source Center, created in 2005 to organize the intelligence community's work based on openly available information, cautions in the report that the information is "based exclusively on the content and behavior of selected media and has not been coordinated with other U.S. government components."

The report is marked as "For Official Use Only" and was obtained and released by the Federation of American Scientists' Secrecy News publication, which provides access to documents on secrecy, intelligence and national security policy.

A U.S. government official confirmed that the report was from the Open Source Center. The official spoke on condition of anonymity because of the sensitivity of the issue.

Among the subtle hints at the future succession, the report says, is North Korean media coverage of Kim Jong Il's 67th birthday in February, "when the regime seemed to go out of its way to underscore the need for continued Kim family rule based on Kim Jong Il's own progeny."

The report also noted "another possible nod" to Kim Jong Un as successor in a Pyongyang television broadcast, also from February, of a children's program called "Good Heart of the Third Child." The show, the report said, "emphasized the moral virtue of the youngest of three brothers in his adherence to socialist principles."

The detailed report provides a Kim family tree _ "Three Generations of Kims" _ complete with available photos, and uses the only known photograph of Kim Jong Un, taken at age 10, to generate an age-progression photo of what the apparent successor may look like at 26.

Kim Jong Il is believed to have suffered a stroke nearly a year ago. He recently appeared in public looking haggard and thin, and South Korean media has reported that he has pancreatic cancer and less than five years to live.

Reports of Kim's ill health spark fears that his death could trigger chaos or a power struggle if he does not name a successor.

Speculation has mounted that recent North Korean aggression _ including a rocket launch in April over Japan, a nuclear test in May and a series of missile tests _ is designed to build up national pride as Kim prepares to name his youngest son as heir.