Pyongyang has replaced its public security minister who has disappeared from public view after a train blast in April that killed 161 people. There is lingering speculation the incident was linked to an assassination attempt on the North's leader Kim Jong-Il.

Choe Ryong-Su had been dismissed from his post just one year after he assumed the important position that controls the country's powerful secret police, a backbone of Kim's totalitarian rule.

South Korean officials said Choe's fate is unknown. Tokyo Radiopress reported that Choe has not been seen since the disaster in Ryongchon, near the Chinese border.

The real question is whether Choe has been executed along with a number of others who may have been involved in a plot to assassinate Kim. The train enroute from China passed through the station anywhere from 30 minutes to nine hours before the blast.

Pyongyang watchers in Seoul had considered Choe to be one of the next generation of North Korean leaders because he replaced Vice Marshal Park Hak-Rim, 86, one of the most influential figures in the North's military. He also replaced Park as the member of the powerful National Defense Commission chaired by Kim Jong-Il.

The North's media gave no explanation for Choe's replacement. But South Korean intelligence officials said Choe was likely fired due to the train blast.

According to diplomatic sources, North Korea's state security agency has concluded that the train explosion was a botched attempt to harm Kim. A mobile phone was reportedly used to spark the explosion.

North Korea watchers said there is a possibility that Choe was punished or executed. Government officials in Seoul declined to comment about Choe's fate, but said he has been absent from public appearances since the train explosion.

Ju Sang-Song, an army general, replaced Choe in a shakeup ordered by the North's legislature, the Supreme People's Assembly on July 9, according to the North's official Korean Central News Agency.

Ju was promoted to the People's Army's full-star general in February 1997. He worked as a member of the North's legislature in 1990 and 1998. His most recent post, as reported by the North's media, was as a front-line army corps commander.

Another source said the replacement was part of an effort to purge close aides to Jang Song-Taek, Kim's brother-in-law, widely considered a candidate for the country's next leader.

Jang, husband of Kim Jong-Il's sister, Kim Gyong-Hi, has lived under house arrest following a bitter internal power struggle, a South Korean intelligence official said.

Jang was recently arrested on charges of "creating a faction" and "peddling his power for benefits" and has led a tightly guarded life at a guesthouse in a suburb of Pyongyang.

"Jang has been dismissed from the key post by those who support one of Kim's sons to be the next country," the source said.