Reports say the US and South Korea have held a ballistic missile drill, after North Korea tested a long-range missile experts believe may reach Alaska.
Self-restraint was “all that separated armistice and war” and could be changed at any time, the two allies said.
It would be a “grave mistake” for the North to think otherwise, they said.
China and Russia have urged the North to suspend its weapons programme in exchange for a halt to US-South Korean military exercises.
The launch, the latest in a series of tests, was in defiance of a ban by the United Nations Security Council (UNSC).
The US has asked for an urgent meeting of the UNSC to discuss the issue. A closed-door session of the 15-member body will take place later this Wednesday.
Pyongyang claimed on Tuesday to have successfully tested an intercontinental ballistic missile (ICBM).
However, most experts believe that the North does not yet have long-range nuclear weapon capabilities.
The two Koreas are technically still at war as the 1950-1953 Korean War ended in an armistice.