North Korea is apparently accelerating its development of missiles and nuclear weapons in response to joint efforts by the United States and South Korea to strengthen their deterrence.

North Korea showcased its missiles -- including a Hwasong-17 intercontinental ballistic missile -- in a military parade on February 8.

New types of missiles on large 18-wheel mobile launchers were also seen. Analysts say these may include a new solid-fuel ICBM.

Pyongyang issued a statement on Friday criticizing the United States and South Korea for planning what it called the largest-ever joint military exercises. It warned that the two nations will "face unprecedentedly persistent and strong counteractions."

Kim Yo Jong, the sister of North Korean leader Kim Jong Un, said in a statement on Sunday, "We will watch every movement of the enemy and take corresponding and very powerful and overwhelming counteraction against its every move hostile to us."

The US and South Korean air forces on Sunday conducted a joint exercise following the North's launch of an ICBM the previous day. The exercise included a B-1 bomber, which Pyongyang is deeply wary of.

The US and South Korea also plan to conduct a tabletop exercise on Wednesday based on the scenario of nuclear threat from North Korea. That will be followed by a regular joint military drill in mid-March.

Japan's Defense Ministry has informed the coast guard of missile launches by North Korea.

Multiple government sources say the missiles appear to have fallen outside of Japan's exclusive economic zone.

Officials are gathering information on the launches.