SEOUL, South Korea (AP) — A latest missile launch by North Korea on Wednesday appeared to have ended in a failure, South Korean defense officials said, three days after the North claimed a major breakthrough in its rocket development program.

The reported launch failure come as U.S. and South Korean troops were conducting their annual military drills that the North calls an invasion rehearsal. Earlier this month, North Korea fired four ballistic missiles that landed in waters off Japan, triggering strong protests from Seoul and Tokyo.

On Wednesday morning, the North fired a missile from the eastern coastal town of Wonsan but the launch was believed to have ended in a failure, Seoul's Defense Ministry said in a statement. It said it was analyzing what type of missile North Korea launched.

American officials said earlier this week that the U.S. military expected another North Korean missile launch in the next several days. The officials said the U.S. had increased its surveillance over the North and had detected a North Korean missile launcher being moved, as well as the construction of VIP seating in Wonsan.

The North's state media said Sunday that it had conducted a ground test of a new type of high-thrust rocket engine, which it hailed as a breakthrough for the country's space program. Weapons experts say such technology has applications for the North's ballistic missile program.