The North says its missiles are on standby, and there are now growing concerns that those missiles could carry nuclear weapons. In an effort to reassure allies, Kerry traveled to the region and discussed the North's nuclear potential.
"If Kim Jong Un decides to launch a missile, whether it's across the Sea of Japan or some other direction, he will be choosing willfully to ignore the entire international community," Kerry told reporters. "And it will be a provocation and unwanted act that will raise people's temperatures."
A new intelligence report suggests that North Korea has made significant progress in its nuclear weapons program. It indicates with moderate confidence the North could put a nuclear warhead on a missile, even though it would affect the missile's reliability.
The North has had nuclear weapons for some time, but the ability to put one on a missile was thought to be beyond their technology until now.
Officials believe the North Koreans have a missile prepared for a test that could reach Japan or Guam, and satellites are tracking signs the North is prepared to fire a barrage of smaller missiles too.
If the trajectory of the test missile suggests that it could be a threat to either the U.S. or allies, the military would move to shoot it down from one of nine warships armed with sophisticated ballistic missile defense systems in the Pacific, including two that were moved closer to the Korean peninsula, U.S. officials said, speaking on condition of anonymity.
"It will further isolate (Kim's) country and further isolate his people who are desperate for food and not missile launches," Kerry warned. "They are desperate for opportunity and not for a leader to flex his muscles."
President Barack Obama publicly stated that the U.S. wants to avoid war.
"Now's the time for North Korea to end the kind of belligerent approach that they've been taking and to try to lower temperatures," Mr. Obama said. "Nobody wants to see a conflict on the Korean peninsula."
In North Korea, many people were in the streets preparing for the April 15 birthday of national founder Kim Il Sung. This year's big flower show features an exhibition of orchids built around mock-ups of red-tipped missiles, slogans hailing the military and reminders of perceived threats to the nation.
The Associated Press contributed to this report.