"If confirmed, this is another example of the extreme brutality of the North Korean regime," US State Department deputy spokeswoman Marie Harf said, adding that Washington was following developments closely.
South Korean President Park Geun-Hye has accused Kim Jong-Un of resorting to a "reign of terror" to cement his leadership. Japan's defence minister warned this week that the purge could herald upheaval comparable to China's disastrous Cultural Revolution.
The Kim family has ruled the North for six decades with an iron fist, regularly purging those showing the slightest sign of dissent. Most are executed or sent to prison camps, but the nation has not seen such a high-level execution for decades.
The regime accused Jang of betraying the trust of both Kim Jong-Un, who is aged around 30, and his father - saying he had received "deeper trust" from the younger leader in particular.
The KCNA report portrayed Jang as decadent and corrupt, attempting to dominate national affairs by "stretching his tentacles" into ministries and institutions, and responsible for the failings of the hungry and impoverished nation.
In a nation long ruled under a pervasive personality cult, Jang was also accused of slighting the young leader - not applauding him loudly enough at party meetings and blocking the construction of a mosaic in his honour at a tile factory.
Deriding Jang as "despicable human scum... worse than a dog", the KCNA report said he had attempted to stand in the way of Kim Jong-Un's succession and then targeted him with a planned coup.
"The accused is a traitor to the nation for all ages who perpetrated anti-party, counter-revolutionary factional acts in a bid to overthrow the leadership of our party and state and the socialist system," it said.
North Korea confirmed earlier this week that Jang, who was married to Kim Jong-Il's sister, had been stripped of all posts, and branded him as a drug-using womaniser who squandered millions in state funds at foreign casinos.
During the court questioning, Jang reportedly confessed he attempted to stage a coup by mobilising his associates in the military.
"I was going to stage the coup by using army officers who had close ties with me or by mobilising armed forces under the control of my confidants", KCNA quoted him as saying.
"I attempted to trigger off discontent among service personnel and people when the present regime does not take any measure despite the fact that the economy of the country and people's living are driven into catastrophe."
Jang was seen as helping Kim Jong Un consolidate power after the death of his father, Kim Jong Il, two years ago. Jang was the latest and most significant in a series of personnel reshuffles that Kim has conducted in an apparent effort to bolster his power.
Some analysts see the purge as a sign of Kim Jong Un's growing confidence, but there has also been fear in Seoul that the removal of such an important part of the North's government - seen by outsiders as the leading supporter of Chinese-style economic reforms - could create dangerous instability or lead to a miscalculation or attack on the South.
Tensions are still high on the Korean Peninsula following a torrent of threats in March and April by Kim Jong Un's government against Washington, Seoul and Tokyo, including vows of missile and nuclear strikes and warnings that Pyongyang would restart nuclear bomb fuel production.
Jang was married to Kim Jong Un's aunt, Kim Kyong Hui, the younger sister of Kim Jong Il. He was earlier described by state media as "abusing his power," being "engrossed in irregularities and corruption," and taking drugs and squandering money at casinos while undergoing medical treatment in a foreign country.
- AP
