SEOUL, South Korea (AP) — South Korea’s military says North Korea has fired at least one unidentified ballistic missile toward its eastern sea.

South Korea’s Joint Chiefs of Staff on Sunday did not immediately say what type of missile it was or how far it flew. The launch came a day after South Korean officials said they detected signs that North Korea was preparing to test a missile designed to be fired from submarines.

North Korea has dialed up its testing activities to a record pace in 2022, testing more than 30 ballistic weapons, including its first intercontinental ballistic missiles since 2017, as it continues to expand its military capabilities amid a prolonged stalemate in nuclear diplomacy.

The launch came as the nuclear-powered aircraft carrier USS Ronald Reagan and its strike group arrived in South Korea for the two countries’ joint military exercise to show their strength against growing North Korean threats.

The North Korean threat is also expected to be a key agenda when U.S. Vice President Kamala Harris visits South Korea next week after attending the state funeral in Tokyo of slain former Japanese Prime Minister Shinzo Abe.

The office of South Korean President Yoon Suk Yeol earlier said that he was briefed on possible North Korean preparations for a submarine-launched ballistic test before his flight back home from a visit to Canada.

On Wednesday, 38North, a North Korea-focused website, said its analysis of commercial satellite imagery shows multiple barges and other vessels gathered at the eastern port of Sinpo, where North Korea has a major shipyard building submarines. The report said the North was possibly preparing to launch a new submarine capable of firing ballistic missiles.

North Korea has been pushing hard to acquire an ability to fire nuclear-armed missiles from submarines, which it sees as a key piece in building a nuclear arsenal that can viably threaten its neighbors and eventually the American homeland.

Such weapons in theory would bolster North Korea’s deterrent by ensuring retaliation after absorbing a nuclear attack on land. Ballistic missile submarines would also add a new maritime threat to the North’s growing collection of solid-fuel weapons fired from land vehicles, which are being developed with an apparent aim to overwhelm missile defense systems in South Korea and Japan.

Still, experts say the heavily sanctioned nation would need considerably more time, resources and major technological improvements to build at least several submarines that could travel quietly in seas and reliably execute strikes.

South Korea’s military in March detected the North flight-testing a ballistic missile from a submarine in March that flew 600 kilometers (372 miles) before landing in waters between the Korean Peninsula and Japan.

The March launch was North Korea’s first testing of a submarine-launched ballistic missile system since October of last year, when it fired a new short-range missile from the 8.24 Yongung – its only known submarine capable of launching a missile. The October underwater launch was the North’s first in two years.