He denied reports that South Korea had raised its alert level by one step to Level 2 from Level 3 on Wednesday in response to Pyongyang's possible preparations to fire a ballistic missile.

Yoon said the alert level, dubbed Watchcon, was upgraded on March 5 when North Korea announced it would void the Armistice Agreement that halted the 1950-53 Korean War.

"The Korea-US intelligence surveillance assets have been bolstered since the (North's) Supreme Command of the Korean People's Army issued a statement," Yonhap quoted Yoon as saying. "It is not true that it was raised yesterday," he said.

South Korean Foreign Minister Yun Byung-se said on Wednesday Pyongyang had moved an intermediate-range Musudan ballistic missile to its east coast and was prepared to launch the missile "at any time from now."

The South Korean Defense Ministry said North Korea could fire a missile around April 15, the anniversary of North Korea founder Kim Il Sung’s birth, when dramatic displays of military power are likely.

Many analysts think Pyongyang's actions are more related to new leader Kim Jong Un's need to prove his credentials with his own military hardliners and extract concessions from the international community rather than any real desire for a conflict with foreign powers.