SEOUL, South Korea — American and South Korean troops increased alert levels on Wednesday as South Korea’s foreign minister warned that North Korea could launch its medium-range Musudan missile “any time from now.”

Although North Korea has tested many of its short-range Scud and medium-range Rodong missiles, it has never flight-tested the longer-distance Musudan, believed by experts to have a range of 1,550 miles to 2,500 miles. A successful test of the missile would demonstrate the North’s potential to hit not only South Korea but also all of Japan and targets as far away as the American military bases on Guam, a Pacific island.

“Based on intelligence we and the Americans have collected, it’s highly likely that North Korea will launch a missile,” Foreign Minister Yun Byung-se of South Korea said at a parliamentary hearing on Wednesday. He added that such a test would violate United Nations resolutions banning the country from testing ballistic missiles. “Such a possibility could materialize at any time from now.”

American and South Korean troops raised their level of vigilance, stepping up monitoring and intelligence-gathering activities, officials at the South Korean Defense Ministry said.