North Korean Foreign Minister Ri Su Yong has warned his country had the power to deter an "ever-increasing nuclear threat" by the US with a pre-emptive strike if necessary.

In a rare speech to the UN-backed Conference on Disarmament, Mr Ri said joint military exercises being staged by South Korea and the United States were "unprecedentedly provocative in nature and have an especially high possibility of sparking off a war".

"The DPRK (Democratic People's Republic of Korea) cannot but bolster its nuclear deterrent capability to cope with the ever-increasing nuclear threat of the US. Now the DPRK has the power of deterring the US and conducting a pre-emptive strike as well, if necessary," Mr Ri told the Geneva forum.

His remarks were consistent with Pyongyang's stance on the annual US-South Korean military exercises, which the North denounces as a preparation for war, and with the verbal threats that the isolationist government frequently makes.

North Korea fired two short-range missiles off its eastern coast on Monday, South Korean officials said, a defiant response to the military exercises that drew a swift protest from Japan.

Mr Ri said the Korean Peninsula was a "touch-and-go nuclear powder-keg".

Striking a conciliatory note, he noted North Korean leader Kim Jong Un had underscored in his New Year's address that the North and South should achieve great national unity.

"The DPRK will not spare its sincere efforts to bring about great change in inter-Korean relations this year," Mr Ri said.