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, writes BBC.
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.
Shortly before the latest launches, the US president spoke of a letter he had received from the North Korean leader.
"It was a very positive letter," Mr Trump told reporters at the White House. "I think we'll have another meeting. He really wrote a beautiful, three-page - I mean great from top to bottom - a really beautiful letter."
The North Korean regime has expressed anger over the US-South Korean military drills, stating that they violate agreements reached with Mr Trump and South Korea's President Moon Jae-in.
While the main drills will start on 11 August, low-key preparation has begun.
The exercises are mainly computer-simulated and are more low key than previous drills between the US and South Korea, which the North has deemed as provocations.
Over recent weeks, the North has test-fired what South Korean officials said appeared to have been a new type of short-range missile, with the previous one coming on Tuesday.
The missiles fired on 25 July - one of which travelled about 690km - were the first since Mr Trump and Mr Kim held an impromptu meeting in June at the demilitarised zone (DMZ), an area that divides the two Koreas.