North Korea routinely test-launches short-range missiles. But the latest launches came during a period of tentative diplomacy aimed at easing recent tension, including near-daily threats by North Korea to attack South Korea and the U.S. earlier this year. North Korea protested annual joint military drills by Seoul and Washington and U.N. sanctions imposed over its February nuclear test.

The fourth launch occurred Sunday afternoon, according to officials at Seoul's Defense Ministry and Joint Chiefs of Staff. The officials, who spoke on condition of anonymity citing department rules, refused to say whether it was a missile or artillery round.

On Saturday, North Korea fired two short-range missiles in the morning and another in the afternoon. The U.S. responded by saying threats or provocations would only further deepen North Korea's international isolation, while South Korea called the launches a provocation and urged the North to take responsible actions.

The North has a variety of missiles but Seoul and Washington don't believe the country has mastered the technology needed to manufacture nuclear warheads that are small and light enough to be placed on a missile capable of reaching the U.S.

U.S. officials said the North has recently withdrawn two mid-range "Musudan" missiles believed to be capable of reaching Guam after moving them to its east coast during the recent tensions.

The Korean Peninsula officially remains in a state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. South Korea's Defense Ministry said Sunday it has deployed dozens of Israeli-made precision guided missiles on front-line islands near the disputed western sea boundary as part of an arms buildup begun after a North Korean artillery strike on one of the islands in 2010 killed four South Koreans.

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