August 2012 photo from China's Xinhua News Agency, Jang Song Thaek, North Korea's vice chairman of the powerful National Defense Commission, attends a meeting on developing economic zones in North Korea. (Li Xin/AP)

The powerful uncle of North Korean supreme leader Kim Jong Un has been “very likely” removed from all his positions, two South Korean lawmakers said Tuesday, in the first suggestion of major upheaval since Kim’s ascension two years ago.

The dismissal of Jang Song Thaek could not be independently confirmed, and North Korea’s state-run media did not indicate that any changes had taken place among its top leadership.

The South’s National Intelligence Service briefed a committee of lawmakers Tuesday about its assessment, also telling them that two of Jang’s aides had been publicly executed, according to a pair of lawmakers who attended the briefing.

Jang, 67, has served as a key regent for Kim and has held one of the four vice chairmen positions on the National Defense Commission, the North’s highest decision-making body.

But North Korea’s state-run media, which carefully record the public appearances of top officials, have not reported on Jang in nearly a month.

Jang has not been seen with Kim since Oct. 10, when both attended a musical performance commemorating the ruling Workers’ Party. In 2011 and 2012, Jang appeared in public alongside Kim several times a week, a significant marker in a secretive country where power is measured by proximity to the leader, experts say.

If Jang has indeed been removed, it would mark the boldest shake-up of top leadership under Kim, who took power after his father, Kim Jong Il, died of a reported heart attack in December 2011. Under the young third-generation leader, the North has made wholesale changes in its military and bureaucracy, but Kim’s inner circle has largely remained intact — though with a few additions.

Jang’s dismissal suggests that “Kim Jong Un’s grab on power is strong, and the competition to be loyal to him is becoming fiercer,” said Cheong Seong-chang, a North Korea researcher at Seoul’s Sejong Institute.

Kim holds nearly every top title in North Korea’s party and military, redundant reminders of his control. There have been no outward signs of instability in Pyongyang under Kim, although the North continues to struggle under its archaic state-run economy. Dissent is categorically banned in the North, and those who criticize the top leadership — or the state doctrine — are sent to one of the North’s gulags or reeducation camps.

Some experts were skeptical about the report of Jang’s dismissal and noted that South Korea’s spy agency has a spotty track record in its assessments of the North. Former U.S. assistant secretary of state Kurt Campbell has called North Korea a “black box” that is “probably the hardest target we face in the global arena.”

Jang was one of the most well-traveled North Korean politicians, and he was seen by some as an advocate for modest economic reform. In 2002, during a warmer period of North-South relations, Jang toured the headquarters of Samsung, rode the Seoul subway system and walked through the Coex Mall, an enormous shopping plaza in Gangnam. In August 2012, Jang traveled to Beijing and met with then-Chinese president Hu Jintao.

While Kim Jong Il led the North, Jang was among Kim’s most trusted lieutenants, as well as one of his regular drinking partners, according to experts who study the North Korean leadership. Jang married Kim Jong Il’s younger sister, Kim Kyong Hui, in 1972.

Jang earned several political promotions and ultimately became senior deputy director of the Organization and Guidance Department, where he had oversight of North Korea’s darkest efforts — including public executions — to snuff out resistance.

Jang ran into trouble in the past decade, though the reason remains unclear. Between 2004 and 2006, he never appeared in public, and some researchers say Jang had perhaps been accused of building his own power base. Still, when he resurfaced, Jang regained much the same trust and was installed by Kim Jong Il as a de facto caretaker for a hereditary power transfer.

If Jang has been removed, analysts are likely to focus on his wife and Kim Jong Un’s aunt, who is often described as having power similar to Jang’s. The NIS assessment did not indicate that Kim Kyong Hui had been removed along with her husband.

The Daily NK, an online newspaper based in Seoul and staffed partly by defectors, reported last month that Jang had been losing political clout and that Kim Jong Un no longer consulted him for political advice. Jang had been told to focus on projects that help raise foreign currency, the Daily NK reported. He also had been put in charge of the state sports guidance commission. One unidentified source told the Daily NK that Kim is “listening to the opinions of fewer and fewer people now.”

“If Jang’s dismissal is in fact true, we can interpret it as Kim Jong Un removing a figure who got in the way of his direct ruling system,” said Kim Yong-hyun, a professor at Dongguk University in Seoul. “Kim Jong Un could have felt his presence and influence burdensome.”

Yoonjung Seo in Seoul contributed to this report.