North Korea already has been punished in recent months for launching a long-range rocket in December and conducting an underground nuclear test in February.
Analysts do not believe North Korea will stage an attack similar to the one that started the Korean War in 1950. But there are concerns that the animosity could spark a skirmish that could escalate into a serious conflict.
"North Korea has been, with its bellicose rhetoric, with its actions ... skating very close to a dangerous line," U.S. Defense Secretary Chuck Hagel said in Washington on Wednesday. "Their actions and their words have not helped defuse a combustible situation."
The missile that officials believe Pyongyang is readying has been dubbed the "Musudan" by foreign experts after the northeastern village where North Korea has a launch pad. The missile has a range of 3,500 kilometers (2,180 miles) and is designed to reach U.S. military installments in Guam and Japan, experts say.
Bracing for a launch that officials said could take place at any time, Seoul deployed three naval destroyers, an early warning surveillance aircraft and a land-based radar system, a Defense Ministry official said in Seoul, speaking on condition of anonymity in line with department rules. Japan deployed PAC-3 missile interceptors around Tokyo.
But officials in Seoul played down security fears, noting that no foreign government has evacuated its citizens from either Korean capital. U.S. Secretary of State John Kerry was headed to Seoul on Friday for talks with South Korean officials on the tensions.
"North Korea has continuously issued provocative threats and made efforts to raise tension on the Korean peninsula ... but the current situation is being managed safely and our and foreign governments have been calmly responding," Foreign Ministry spokesman Cho Tai-young told reporters Thursday.