South Korea's military is reportedly keeping close tabs on North Korea, weighing the possibility that the "new strategic weapon" North Korean leader Kim Jong-un warned of could be an intercontinental ballistic missile(ICBM) carrying multiple warheads.

A military official said on Thursday that Seoul believes Pyongyang could have upgraded its Hwasong-15 ICBM following two rounds of engine tests it conducted late last year, suggesting the possibility of an ICBM supporting a multiple independently targetable reentry vehicle.

Another official said Pyongyang's new strategic weapon could also be a submarine-launched ballistic missile or a three-thousand-ton-class submarine currently ready for an inaugural launch.

Military officials say the North will need to have secured technology for what is called a "post boost vehicle" to precisely position and release multiple warheads to accurately strike their targets.

An official said it's unclear whether the North has secured such technology.