North Korea has declared a no-sail and no-fly zone off its east coast in the Sea of Japan, South Korean media reported Monday, suggesting more missile launches are possible before the U.S. defense chief visits Seoul this week.

Pyongyang fired four short-range missiles off its west coast Friday in what South Korea called a bid to stoke tension during its annual joint military drills with the United States.

The two Koreas are locked in a tense standoff, trading harsh rhetoric recently over the arrest by Pyongyang of two South Korean nationals it accused of espionage.