South Korean Defense 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 added that there were no signs that Pyongyang was preparing for a full-scale conflict.
The U.S. says it is poised to respond, despite doubts over the North's ability to pull off the threat. North Korea's military warned it has been authorized to attack using "smaller, lighter and diversified" nuclear weapons, but the North is not believed to have mastered the technology needed to miniaturize nuclear bombs enough to mount them on long-range missiles.
The Pentagon is surrounding the Korean peninsula with layers of missile defense, sending two guided missile destroyers and land-based interceptors to Guam.
"Some of the actions they've taken over the last few weeks present a real and clear danger and threat" to the U.S. and its allies, U.S. Defense Secretary Chuck Hagel said Wednesday.
Some experts say North Korea might be considering a test launch April 15 -- the birth date of the country's founder and leader Kim Jong Un's grandfather.
There is concern about South Korea, where more than 28,000 U.S. troops are stationed. Officials say Pyongyang may be initiating a small-scale provocation against the South similar to an exchange of fire in 2010 on Yeonpyong Island that killed four people.
A team of U.S. chemical weapons experts returned to South Korea Thursday for the first time in eight years. The drill is meant to reassure residents an attack can be contained.
North Korea has been railing against joint U.S.-South Korean military exercises that are taking place in South Korea and has expressed anger over tightened U.N. sanctions for its February nuclear test.
The Associated Press contributed to this report.