SEOUL (NYTIMES) - The last time South Korea is known to have plotted to assassinate the North Korean leadership, nothing went as planned.

In the late 1960s, after North Korean commandos tried to ransack the presidential palace in Seoul, South Korea secretly trained misfits plucked from prison or off the streets to sneak into North Korea and slit the throat of its leader Kim Il Sung. When the mission was aborted, the men mutinied.

Now, as Mr Kim's grandson, Mr Kim Jong Un, accelerates his nuclear programme, South Korea is again targeting the North's leadership. A day after North Korea conducted its sixth - and by far most powerful - nuclear test this month, South Korean Defence Minister Song Young Moo, told lawmakers in Seoul that a special forces brigade he described as a "decapitation unit" would be established by the end of the year.

The unit, officially known as the Spartan 3000, has not been assigned to literally decapitate North Korean leaders. But that is clearly the menacing message South Korea is trying to send.

Mr Song said the unit could conduct cross-border raids with retooled helicopters and transport planes that could penetrate North Korea at night.

Rarely does a government announce a strategy to assassinate a head of state, but South Korea wants to keep the North on edge and nervous about the consequences of further developing its nuclear arsenal.

At the same time, the South's increasingly aggressive posture is meant to help push North Korea into accepting President Moon Jae In's offer of talks.

"The best deterrence we can have, next to having our own nukes, is to make Kim Jong Un fear for his life," said Mr Shin Won Sik, a three-star general who was the South Korean military's top operational strategist before he retired in 2015.

South Korea has now introduced three arms-buildup programmes - Kill Chain; the Korea Air and Missile Defence programme; and the Korea Massive Punishment and Retaliation initiative, which includes the decapitation unit.

As word of South Korea's new assassination plans has spread, Mr Kim has used his deputies' cars as decoys to move from place to place, South Korean intelligence officials told lawmakers in June.