North Korea launched several cruise missiles into the sea from the country's eastern coast on Wednesday morning, the South's military said, as the nuclear-armed country continues to flex its military muscle.

In a statement, the South Korean Joint Chiefs of Staff said that an undetermined number of cruise missiles had been spotted flying over waters off the northeastern city of Wonsan around 9 a.m.

"While strengthening our monitoring and vigilance, our military has been closely coordinating with the United States to monitor additional signs of North Korea's provocations," it said, adding that it was continuing to analyze the launches with U.S. intelligence agencies.

The launches were Pyongyang's fifth known barrage of cruise missiles since mid-January and part of a spate of weapons tests by the North since the start of the year. These have included new strategic cruise missiles and submarine-launched weapons designed to carry small nuclear bombs.

Experts say cruise missiles present a unique danger in that they can fly low and maneuver, making them potentially very difficult to intercept by air and missile defenses. Unlike ballistic weapons, North Korea’s cruise missiles are not banned under United Nations sanctions imposed on Pyongyang.

In recent months, North Korean leader Kim Jong Un has stepped up his rhetoric against the South, labeling Seoul Pyongyang's "primary enemy" and abolishing agencies focused on reunification, while also threatening to enshrine in his country's constitution a goal of “completely occupying, subjugating and reclaiming” its southern neighbor.

The unusual rhetorical shift has even prompted some longtime North Korea observers to say that Kim, disillusioned with diplomacy, is girding for conflict.