"He was accused of spying for the United States for poorly reporting on the negotiations without properly grasping US intentions," the source was quoted as saying. The summit in Hanoi, the second between Kim Jong-un and Trump, failed to reach a deal because of conflicts over US calls for complete denuclearization of the Korean Peninsula and North Korean demands for sanctions relief.

Kim Yong-chol, Kim Jong-un's right-hand man and the counterpart of US Secretary of State Mike Pompeo before the Hanoi summit, had also been sent to a labour and re-educationcamp near the Chinese border, the Chosun Ilbo reported.

Key officials who worked with Kim Yong-chol have also been out of the public eye since the summit, while seasoned diplomats who had been sidelined were seen returning to the spotlight. A South Korean lawmaker told Reuters in April that Kim Yong-chol, a hawkish former spymaster, had been removed from a key party post. 

A spokeswoman at Seoul's Unification Ministry declined to comment on the Chosun report. An official at the presidentialB lue House in Seoul said it was inappropriate to comment on an issue that should first be verified.

Hong Min, a senior fellow at the Korea Institute for National Unification in Seoul, said it would not be strange if Kim Hyok-chol had been executed because he was a key figure who reported directly to Kim Jong -un. "His task was to convey US views accurately to Kim, but I'm not sure if he was capable of grasping their intentions or negotiating strategy," Hong said.

Kim Hyok-chol was seen as a rising star when he was appointed to spearhead working-level talks with US nuclear envoy Stephen Biegun weeks before the Hanoi summit. However, little was known about his expertise or the role he undertook during those talks. 

The other four executed alongside him included diplomats working on Vietnam relations, the Chosun report said. Kim Song-hye, who led preparations as part of Kim Yong-chol's team, and Sin Hye-yong, a newly elevated interpreter for the Hanoi summit, were also said to have been detained in a camp for political prisoners, the newspaper said. Kim Song-hye had also worked closely with Kim Yo-jong, the North Korean leader's younger sister and a senior party official whom Kim Song-hye accompanied to South Korea for the Winter Olympics last year.

Kim Yo-jong was also lying low, the paper reported, citingan unidentified South Korean government official.Sin was charged with making critical interpretation mistakesthat included missing an unspecified "last-minute offer" theNorth Korean leader supposedly made as Trump was about to walkout, Chosun reported.

North Korea's official party mouthpiece Rodong Sinmun warned on Thursday that "two-faced" officials would face the "stern judgment of the revolution"."It is an anti-party, anti-revolutionary act to pretend to be revering the leader in front of him when you actually dream of something else," it said in a commentary."There are traitors and turncoats who only memorize words of loyalty toward the leader and even change according to the trendof the time," it said. 

The newspaper accused Jang Song-thaek, Kim Jong-un's uncle,o f committing "anti-party, anti-revolutionary acts" after he was executed in December 2013. (Reuters)