Mr. Kim’s order, which North Korea said was given during an emergency meeting early Friday, was similar to the one issued Tuesday when the North’s top military command told all its missile and artillery units to be on the “highest alert” and ready to strike the United States and South Korea in retaliation against their joint military exercises.

But by attributing such an order to its top leader, North Korea tried to add weight to its threat.

“We believe they are taking follow-up steps,” said Kim Min-seok, spokesman of the South Korean Defense Ministry, referring to increased activities of the North Korean military units. "South Korean and American intelligence authorities are closely watching whether North Korea is preparing its short, medium, and long-range missiles, including its Scud, Rodong and Musudan.”

He did not elaborate. But government officials and South Korean media said that there had been a surge in vehicle and troop movements at North Korean missile units in recent days as the United States and South Korea has been conducting joint military drills. The national news agency Yonhap quoted an anonymous military source as saying that North Korean vehicles had been moving to Tongchang-ri near the North’s western border with China, where its Unha-3 rocket blasted off in December.

North Korea might be preparing for an engine test ahead of a long-range rocket test, the source was quoted as saying. Scud and Rodong are the North's mainstay short- and medium-range missiles. The Musudan, deployed around 2007 and displayed for the first time during a military parade in the North Korean capital, Pyongyang, in 2010, is a road-mobile intermediate-range ballistic missile with a range of more than 1,900 miles, according to the South Korean Defense Ministry.

In an angry reaction to the sanctions that the United Nations imposed after North Korea’s launching of a three-stage rocket in December and its third nuclear test last month, the North has repeatedly threatened to strike Washington, as well as the American military bases around the Pacific and in South Korea, with nuclear-armed long-range missiles.