North Korea has fired two unidentified missiles off its east coast, its fourth such launch in less than two weeks, South Korea's military has said.
The projectiles were fired early on Tuesday from South Hwanghae province, the South's Yonhap news agency said.
The US said it was monitoring the situation and consulting with South Korea and Japan.
North Korea has expressed anger at US-South Korean military exercises that got under way on Monday.
The annual drills have been kept low key but North Korea says they violate agreements reached with President Donald Trump and South Korea's President Moon Jae-in.
In a statement, the North's foreign ministry accused the two allies of "playing all sorts of tricks" to justify the military exercises and said their "aggressive nature" could not be covered up.
North Korea has been the subject of a series of US and international sanctions over Pyongyang's development of nuclear weapons and a series of missile tests.
What were the earlier launches?
Last Friday the North test-fired what South Korean officials said appeared to have been a new type of short-range missile.
Those two missiles landed in the Sea of Japan, also known as the East Sea, according to South Korea's Joint Chiefs of Staff (JCS).
The JCS said the missiles flew very low and travelled about 220km (140 miles). Analysts said they appeared to have been unusually fast.
On Wednesday, the North launched two missiles that flew 250km and reached a height of 30km before landing in the Sea of Japan, according to South Korea.
On 25 July, the North had fired two other missiles, one of which travelled about 690km.
That was the first launch since President Trump and North Korean leader Kim Jong-un held an impromptu meeting in June at the demilitarised zone (DMZ), an area that divides the two Koreas, where they agreed to restart denuclearisation talks.