North Korea has fired a projectile that may be a ballistic missile, Japan’s coast guard said Sunday.

The projectile has likely already fallen, the coast guard said, while warning ships to not approach they get near it.

The launch comes after North Korea this month set off one of its biggest series of missile volleys since August 2019. It also threatened to end its nearly four-year hiatus on tests of nuclear weapons and long-range missiles designed to deliver warheads to the US mainland.

North Korean leader Kim Jong Un told a top-level meeting of his ruling party at the end of 2021 that he was more interested in bolstering his arsenal than returning to nuclear disarmament talks with the US, which have been stalled for nearly three years.

While Kim has stayed away from such talks, he has been steadily increasing his nuclear arsenal, testing new systems for quick strikes and those that can maneuver in flight to avoid U.S.-operated interceptors in the region. These have included long-range cruise missiles that could hit almost all of Japan, a new submarine-launched ballistic missile and a hypersonic system designed to use high speeds and maneuverability to deliver a warhead.

North Korea has also resumed plutonium-producing operations at its main Yongbyon nuclear site last year, while satellite imagery shows it expanding a plant that enriches uranium for weapons.

The latest launch provides a reminder to the Biden administration that Kim’s nuclear arsenal remains among the US’s biggest foreign policy challenges despite former President Donald Trump’s decision to hold face-to-face summits with the North Korean leader. Although Kim made a vague commitment in 2018 to “work toward complete denuclearization of the Korean Peninsula,” he has continued to advance his nuclear weapons program.

The North Korean leader is facing one of the most difficult periods during his decade in power. Sanctions have helped push his economy to its smallest since he took charge and his closure of borders about two years ago due to the coronavirus slammed the brakes on the little trade he had.

But Kim’s regime has also found ways to evade sanctions, with the US and United Nations Security Council accusing it of stepping up its cybercrimes to fill its depleted coffers.