North Korea has fired two missiles into the sea, its fifth such launch in recent weeks.
The missiles are thought to be short-range ballistic missiles, South Korea’s military says.
If confirmed, such a test would be a breach of 11 UN security Council resolutions.
The launches come after US President Donald Trump said he had received a “very beautiful letter” from North Korean leader Kim Jong-un.
Mr Trump said Mr Kim was unhappy with the current US-South Korea joint military exercises.
The missiles were fired from near the eastern city of Hamhung in South Hamgyong Province and landed in the Sea of Japan to the east of the Korean peninsula.
Launched at 05:34 and 05:50 (20:34 and 20:50 GMT Friday), they flew a distance of around 400 km (250 miles), at an altitude of about 48 km with a maximum speed of more than Mach 6.1, a South Korean military statement said.
North Korea has fired a series of missiles and rockets since Mr Trump and Mr Kim agreed during a meeting in June to restart denuclearisation negotiations.
