US President Barack Obama has dismissed North Korea's proposal to suspend nuclear tests if the US ends its annual military exercises with the South.
On Sunday Mr Obama told reporters that the US did not take such a proposal seriously and that Pyongyang would "have to do better than that".
The North's foreign minister Ri Su-yong made the offer in a rare interview.
Annual military drills conducted by the US and South Korea routinely inflames tensions with the North.
Mr Ri's comments came as the North said it fired a ballistic missile from a submarine off its eastern coast.
The United Nations condemned the test, which it called a "serious violation" of past resolutions aimed at curbing the North's nuclear ambitions.