The announcement came only days after Pyongyang announced through state media that Jang Song Thaek -- long considered the country's No. 2 power -- had been removed from all his posts because of allegations of corruption, drug use, gambling, womanizing and leading a "dissolute and depraved life."
State news agency KCNA said a tribunal examined Jang's crimes, including "attempting to overthrow the state by all sorts of intrigues and despicable methods with a wild ambition to grab the supreme power of our party and state."
The report called him "a traitor to the nation" and "worse than a dog."
National Security Council spokesman Patrick Ventrell said that there was no reason to doubt the report of Jang’s death and if true, it illustrated North Korea's "extreme brutality."
"While we cannot independently verify this development, we have no reason to doubt the official KCNA report that Jang Song Thaek has been executed,” Ventrell said. “If confirmed, this is another example of the extreme brutality of the North Korean regime. We are following developments in North Korea closely and consulting with our allies and partners in the region."
Jang was seen as helping Kim Jong Un consolidate power after the death of his father, Kim Jong Il, two years ago.
Some analysts see North Korea's personnel reshuffle 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.