Another round of missile tests was carried out by North Korea on Saturday (Picture: KCNA).
North Korea has fired two of what are thought to be short-range missiles on Saturday, South Korean military said.
If confirmed, such a test would be a breach of 11 UN Security Council resolutions.
The South Korean Joint Chiefs of Staff said in a statement more missile launches are highly probable as the North is conducting its own summer drills.
The launches came just hours 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.
A US official has reportedly said that at least one projectile was launched and that it appeared to be similar to previous short-range missiles fired by Pyongyang.
Two missiles flew about 400 km (250 miles) at a height of around 48 km, according to the South Korean military.
Earlier this week, North Korea fired two unidentified projectiles into the East Sea.