SEOUL (Reuters) - North Korea fired on Monday what could be a ballistic missile, Japan's coast guard said, in what would be the fourth test this month as Pyongyang forges ahead with new military developments amid stalled stalks with the United States and South Korea.

       South Korea's military also reported that the North had fired an "unidentified projectile" toward the ocean off its east coast.

       Since New Year's Day, North Korea has conducted three other tests, an unusual frequency of weapons tests. Two of those launches involved single "hypersonic missiles" capable of high speeds and manoeuvring after launch, while the last, on Friday, involved a pair of short-range ballistic missiles (SRBMs) fired from train cars.

       It was not immediately known what kind of missile was involved in Monday's reported launch.

       The series of launches prompted U.S. President Joe Biden's administration to impose its first sanctions against Pyongyang on Wednesday, and to call on the U.N. Security Council to blacklist several North Korean individuals and entities.

       North Korea has defended the missile tests as its sovereign right to self-defence and accused the United States of intentionally escalating the situation with new sanctions.

       (Reporting by Josh Smith; Editing by Christopher Cushing and Neil Fullick)