As well as the obvious classes like bomb-making, Kim Dong-Sik's intensive spy training in North Korea included memorising hundreds of South Korean pop songs and dance moves.

Such strategies for assimilating into South Korean society were considered essential to avoid suspicion, Kim revealed in a recently-published memoir that lifts the lid on some of the workings of the North's spy agency.

Now 51, Kim was captured in the South in 1995 and spent years undergoing interrogation before renouncing communism and joining the South's intelligence authorities as an analyst.

In his memoir, he recalls being handpicked when he was 17, and entering the Kumsong political military university in Pyongyang which specialised in nurturing secret agents to run covert operations in the South.

He was among 200 students selected each year after a national search that took into account looks, family background, school grades and, above all, unquestioning loyalty to the North Korean leadership.

The elite trainees were not allowed to leave the campus or contact anyone outside its walls -- including their families.

The only exceptions were New Year greetings cards they were allowed to send home, but with no return address.

Days were filled with intense training in a large range of skills from martial arts, weapons and bomb-making to wall climbing, geology, Morse code and marine navigation.

All these were fitted around mandatory ideology classes.

In an interview in Seoul with AFP, the stocky, soft-spoken Kim said the course instructors repeatedly hammered home the idea that he and his classmates must always be prepared to sacrifice their lives for their mission.

If faced with capture, they should swallow cyanide pills rather than allow themselves to be taken alive and interrogated.

"The thought of death always weighed on our shoulders ... it was a very heavy burden for a bunch of 20-year-olds," he said.

Dozens dropped out of the high-pressure course, but Kim persevered, lured in part by the prospect of quick promotion within the ruling Workers' Party.

After graduation, the focus switched to training the agents to pass as locals.

South Koreans abducted and smuggled back into the North were among those who instructed them in mastering the right accents and understanding the social and political culture of the capitalist South.

This "enemization" process gave them their first real taste of life outside the isolated North, as they consumed a daily diet of South Korean TV shows, movies, magazines, newspapers and books.

Popular songs and dance moves were memorised, along with the names and careers of prominent TV celebrities and sports stars.

The course material suggested that the life of South Koreans was very different from the image portrayed in the North's official propaganda of "impoverished puppets suffering under US imperialism".

But Kim said he and the other trainees saw nothing amiss.

"We were too loyal to be shaken up by things like that," he said. "We were told that only rich capitalists could enjoy all the good things in the South, and we never questioned that."

After nearly 10 years of training, Kim was finally given his first mission and sent to the South in 1990 to try and recruit some left-wing activists, and help bring back Ri Son-Sil, an elderly agent who ran a vast network of spies in the South.

Two years later, Ri's name would make headlines in the South when dozens of leftwing activists were arrested and accused of cooperating with her to spy for the North in the 1980s.

"The activists were important targets because they could provide valuable inside information and help shape public opinion in our favour if they got into politics," Kim said.

His mission was successful and he was awarded the prestigious "Hero of the Republic" medal on his return to North Korea.

In 1995, he was sent back as part of a two-man team to bring out -- by force if necessary -- a spy posing as a monk who was suspected of working as a double agent with the South Korean intelligence service.

The mission was compromised and the two were tracked down by South Korean security forces. In the ensuing gun battle, Kim's fellow agent was killed and he was wounded and captured.

Kim declined to comment in any detail about the four years he spent in the custody of military intelligence, saying only that it was "quite difficult."

In the end, he was not imprisoned, but taken on as an intelligence officer having renounced any loyalty to the North.

"Apparently they thought that, with all the experiences and information I had, I had more value as an intelligence analyst than something else," Kim said.

The North Korean authorities were not so forgiving. Kim later learned his parents in the North had been "purged" after his arrest, meaning they had either been sent to a prison camp or executed.

"In their eyes I had failed twice -- both with the mission and in not killing myself before being arrested," he said.

He started a new life in South Korea, marrying and having two sons, and earning a Ph.D with a thesis on the North's spying strategy.

"My life was once full of drama ... there were so many days when death seemed to be constantly hanging over my head," he said.

"But I've learned to appreciate an ordinary, quiet life. I wish I could grow old and die that way."

Kim, who still guards his privacy and refused to be photographed front-on, said he had written the book for his sons, so that "they can someday understand my past."

"I also wanted to get things off my chest ... and put this piece of history people don't really know into the record," he added.