(Adds details, expert comment)
By Ju-min Park and Jack Kim
SEOUL, Dec 9 (Reuters) - North Korea announced on Monday the dismissal of Jang Song Thaek, the once powerful uncle of leader Kim Jong Un, for what it described as a string of criminal acts including mismanaging the economy, corruption, womanising and drug-taking.
The sacking of the man regarded as the second most powerful in the secretive state comes after reports in South Korean media that one of his aides had sought asylum in South Korea.
The unidentified aide, who managed funds for Jang, was being protected by South Korean officials in a secret location in China, cable news network YTN and the Kyunghyang Shinmun newspaper said on Friday, citing sources familiar with the matter.
Jang was removed from all his posts and expelled from the ruling Workers' Party during a meeting of its politburo on Sunday, the North's official KCNA news agency said. Kim Jong Un attended and "guided" the meeting, it said.
"Jang and his followers committed criminal acts baffling imagination and they did tremendous harm to our party and revolution," KCNA said, without saying if Jang had been detained or charged with any crime.
The report also did not refer to Jang's aide, whose defection, if confirmed, would be the most serious for North Korea in 15 years.
South Korea's National Intelligence Service last week said it believed Jang had been relieved of his posts in November. It also said two of Jang's close associates were executed recently for corruption.
The sacking means Pyongyang is undergoing its biggest leadership upheaval since the death in 2011 of former leader Kim Jong Il, the younger Kim's father.
Among Jang's senior party and military posts, he was vice chairman of the country's top military body, the National Defence Commission.
KCNA listed a series of reasons why Jang was dismissed, including mismanagement of the country's financial system, corruption, womanising and abusing alcohol and drugs.
"Jang pretended to uphold the party and leader but was engrossed in such factional acts (such) as dreaming different dreams and involving himself in double-dealing behind the scene," KCNA said.
"Affected by the capitalist way of living, Jang committed irregularities and corruption and led a dissolute and depraved life."
Jang is married to Kim's aunt, the daughter of the North's founding leader Kim Il Sung, and was widely considered to be working to ensure his nephew firmly established his grip on power in the past two years.
Last week a South Korean official said Jang was likely alive and in no immediate physical danger, as was his wife, Kim Kyong Hui.
Experts say Jang's removal will help the younger Kim consolidate his power base with a group of younger aides.
NOT SO LUCKY THIS TIME?
Jang had been a prominent fixture in many of the reports and photographs of Kim Jong Un's public activities, but his appearances have tapered off sharply this year and he has not been since in official media since early November.
He has survived previous purges and official displeasure, thanks largely to his sometimes tempestuous marriage to Kim Kyong Hui, but this time was different, said Jeung Young-tae, an expert at South Korea's Institute for National Unification.
"Jang is gone and purged. In North Korea, there can be no two suns," Jeung said.
YTN said Jang's aide fled to China in late September or early October and that Jang could have been sacked because of this. It said the aide had knowledge of funds belonging to the younger Kim and Kim Jong Il.
South Korea's National Intelligence Service had no knowledge of the defection, lawmakers said on Friday after they were briefed by the head of the spy agency.
China's Foreign Ministry said it had noted the reports, but did "not understand the situation". U.S. national security officials said the United States was aware of the reports but cannot substantiate them.
About 25,000 North Koreans have defected to the South but few of them were highly placed in Pyongyang.
The highest-profile defection was Hwang Jang Yop, a Worker's Party ideologue who was the architect of the Juche (self-reliance) ideology of North Korea. He sought asylum in the South in 1997. (Reporting by Jack Kim and Ju-min Park. Editing by Dean Yates)