The official North Korean news agency said on Wednesday the country's leader Kim Jong Un met his nuclear scientists for a briefing and declared that he was greatly pleased that warheads had been miniaturised for use on ballistic missiles.
In a report dated 9 March, 2016, the KCNA news agency said Kim told the nuclear researchers that the stronger the country's nuclear strike capability, the more effective it will be as a deterrent to aggression.
Pyongyang has previously claimed it has nuclear warheads small enough to put on long-range missiles, but experts have questioned such claims.
North Korea warned on Monday of pre-emptive nuclear strikes after the United States and South Korea began holding their biggest ever war games.
Tensions remain high after North Korea's recent nuclear test and rocket launch, which prompted the United Nations to adopt tough new sanctions.