North Korean top leader Kim Jong-Un is being quoted saying his country's military scientists have successfully miniaturized a thermo-nuclear warhead.

A photo shows Kim Jong-Un meeting with military scientists on thermo-nuclear warhead. [Xinhua]

Kim has reportedly made the comments as part of a meeting with his nuclear development staff.

His comments come a day after the North threatened pre-emptive nuclear attacks on South Korea and the US mainland.

The threats come as US and South Korean forces get large-scale joint military exercises underway this week.

Military tensions have surged on the Korean peninsula since the North's nuclear test and rocket launch earlier this year, which has prompted new sanctions from the UN Security Council.