The Japanese government says North Korea appears to have fired one ballistic missile into the Sea of Japan early on Tuesday.
The South Korean military said the North appears to have fired a projectile. It is collecting and analyzing information with the US military.
North Korea announced earlier this month that it had successfully test-fired newly developed long-range cruise missiles on September 11 and 12.
It also fired two short-range ballistic missiles into the Sea of Japan on September 15. It's likely they fell inside Japan's exclusive economic zone.
Tuesday's launch comes on the same day that North Korea is expected to convene a parliamentary session.
It will be the first meeting of the Supreme People's Assembly since January if it goes ahead.
Last week, Kim Yo Jong, the sister of North Korean leader Kim Jong Un, called on South Korea to abandon its hostility against the North. She also referred to the possibility of holding another inter-Korean summit.