North Korea's foreign minister warned Tuesday Pyongyang had the power to conduct a "pre-emptive strike" on the United States, following joint US-South Korea military drills earlier this week.

Speaking before the UN Disarmament Conference in Geneva, Ri Su-yong said the military exercises that kicked off Monday were "unprecedentedly provocative in nature" and could spark 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," he said.

"Now the DPRK has the power of deterring the US and conducting a pre-emptive strike as well if necessary."

The annual joint exercises always trigger a surge in military tensions and warlike rhetoric on the divided peninsula.

North Korea fired two short-range Scud missiles into the sea off its east coast on Monday, and South Korea's Yonhap news agency reported Tuesday that Pyongyang may be readying to test fire a medium-range missile.

Missile tests have long been a preferred North Korean method of expressing anger and displeasure with what it views as confrontational behaviour by the South and its allies.

Seoul and Washington insist the exercises are defence-based in nature, but they are regularly condemned by Pyongyang as provocative rehearsals for invasion. (AFP)