Pyongyang has miniaturised versions of nuclear warheads that would fit on ballistic missiles, North Korean leader Kim Jong-un reportedly said Wednesday. Kim's claim comes a couple of days after the U.S. and South Korea started their annual joint military drills, which the North said was a call for war and threatened to respond with pre-emptive strikes. 

Kim made the statement while inspecting the nuclear warheads, including the one developed for thermo-nuclear reaction, Reuters reported, citing the Korean Central News Agency (KCNA).

"The nuclear warheads have been standardised to be fit for ballistic missiles by miniaturising them...this can be called a true nuclear deterrent," KCNA quoted Kim as saying.

"He stressed the importance of building ever more powerful, precision and miniaturised nuclear weapons and their delivery means," Reuters cited KCNA.

On Jan. 6, the North said it tested a miniaturised hydrogen bomb, however, international leaders expressed their doubts on Pyongyang's claims, which was not independently verified either. It also launched long-range rocket Feb. 7, despite warnings from the U.S.

While North Korea claimed the rocket launch was to send a satellite into orbit, the U.S. saw it as a test of ballistic missile technology, which is banned under the United Nations Security Council (UNSC) resolutions.

Both the tests prompted the UNSC to adopt tough sanctions against North Korea. The UNSC on March 2 unanimously voted in favour of imposing new sanctions, considered to be the toughest so far, on North Korea. 

The sanctions when implemented will make it mandatory for all cargoes entering and exiting North Korea via the sea, land or air routes to undergo inspections. It will also put a ban on export of coal, iron and other mineral resources that bring hard currency to the nation.

North Korea responded to the UNSC's sanctions by firing short-range missiles from the country's Wonsan port city into the Sea of Japan Thursday.