Pyongyang, March 9 North Korean leader Kim Jong-un has claimed that his scientists have developed miniaturised nuclear warheads that can fit on ballistic missiles, media reports said on Wednesday. The North Korean state media has made this claim before also, but this is the first time Kim has been quoted directly. However, experts have long cast doubt on such assertions from the North, BBC reported. According to news agency Xinhua, The top North Korean leader said nuclear warheads "have been standardized to be fit for ballistic missiles" through miniaturisation. "This can be called true nuclear deterrence," Kim Jong Un was quoted by KCNA news agency when he met nuclear scientists and technicians and guided work for further developing the country's nuclear arsenal. Kim debriefed the scientists on the research about tipping various types of tactical and strategic ballistic missiles with nuclear warheads as well as specifications of Korean-style miniaturized nuclear warheads. The main mission for North Korea's nuclear forces is to deter a nuclear war, he said, adding that the more the nuclear strike capability gets, the more powerful North Korea's deterrent to aggression and nuclear war grows. The state has stepped up its bellicose rhetoric in the wake of tough UN sanctions imposed after its nuclear and rocket tests earlier this year. In recent days it has also threatened to launch an "indiscriminate" nuclear strike, as the US and South Korea began their largest ever round of annual military exercises, known as Foal Eagle and Key Resolve.