After a series of escalating threats, North Korea has moved a missile with "considerable range" to its east coast, South Korea's defence minister has said.
But he emphasised that the missile was not capable of reaching the United States and that there are no signs that the North is preparing for a full-scale conflict.
North Korea has been railing against US-South Korean military exercises that began in March and are to continue until the end of this month. The allies insist the exercises in South Korea are routine, but the North calls them rehearsals for an invasion and says it needs nuclear weapons to defend itself. The North has also expressed anger over tightened UN sanctions for its February nuclear test.
Analysts say the ominous warnings in recent weeks are probably efforts to provoke softer policies from South Korea, to win diplomatic talks with Washington and solidify the image of young North Korean leader Kim Jong Un. Many of the threats come in the middle of the night in Asia - daytime for the US audience.
The movement of the missile came hours after North Korea's military warned that it has been authorised to attack the US using "smaller, lighter and diversified" nuclear weapons. The reference to smaller weapons could be a claim that North Korea has improved its nuclear technology, or a bluff.
The North is not believed to have mastered the technology needed to miniaturise nuclear bombs enough to mount them on long-range missiles. Nor has it demonstrated that those missiles, if it has them at all, are accurate. It could be years before the country completes the laborious process of creating enough weaponised fuel to back up its nuclear threats.
South Korean Defence Minister Kim Kwan-jin said he did not know the reasons behind the North's missile movement, and that it "could be for testing or drills".
He dismissed reports in Japanese media that the missile could be a KN-08, which is believed to be a long-range missile that if operable could hit the US. Kim told lawmakers at a parliamentary committee meeting that the missile has "considerable range" but not enough to hit the US mainland.
The range he described could refer to a mobile North Korean missile known as the Musudan, believed to have a range of 1,800 miles. That would make Japan and South Korea potential targets - along with US bases in both countries - but there are doubts about the missile's accuracy.
The Pentagon announced that it will hasten the deployment of a missile defence system to the US Pacific territory of Guam to strengthen regional protection against a possible attack.