Seoul, South Korea – North Korea has fired at least one short-range ballistic missile into the sea, Yonhap news agency reported Wednesday, hours after an American nuclear-armed submarine made its first South Korean port call in decades.

The launch was reported by South Korea's Joint Chiefs of Staff, according to Yonhap, who said they were still analyzing what type of missile had been fired, and that it had landed in the East Sea, also known as the Sea of Japan.

Japan's Coast Guard said a second missile may also have been launched, according to Kyodo news agency.

The launch is the latest in a series of weapons tests by Pyongyang, and comes as Seoul and Washington ramp up defense cooperation in the face of soaring tensions with the North.

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On Tuesday, they held the first Nuclear Consultative Group meeting in Seoul and announced an American nuclear submarine was making a port visit to Busan for the first time since 1981.

The move had been expected to trigger a strong response from North Korea, which balks at having US nuclear assets deployed around the Korean peninsula.

The launch also comes less than a week after leader Kim Jong-un personally oversaw the firing of the country's newest intercontinental ballistic missile, the solid-fuel Hwasong-18.

Diplomacy between Pyongyang and Seoul has stalled and Kim has called for ramping up weapons development, including tactical nukes.

In response, Seoul and Washington have staged joint military exercises with advanced stealth jets and US strategic assets.

The launch also comes as Washington confirmed Tuesday that a US soldier is believed to have been detained by North Korea after crossing the heavily fortified border.