North Korea has banned vessels from navigating in its waters in one Yellow Sea location through the end of June and in two other areas until the end of next month.

A South Korean official says the ban may just be part of a regular military exercise, but intelligence authorities will continue to keep a close eye on the situation. The official says Seoul fears the ban may signal a provocation by the North or the launch of a short-range missile.

In a Monday briefing, Defense Ministry spokesman Won Tae-jae said if the ban was meant for civilian vessels, Japan would have been notified. But he said Tokyo hasn't made any such official notice.

On reports that the North Korean military ordered the doubling of an ammunition stockpile at a military base and on a patrol ship in the Yellow Sea, the spokesman said it could be part of the North's preparations for military actions.