North Korean leader Kim Jong-un supervised the test-firing and "expressed great satisfaction" with the results.
The projectiles fired on Thursday flew about 380 kilometres (235 miles) at a most altitude of 97 kilometres (60 miles), essentially based entirely on South Korea's Joint Chiefs of Workers.
"Our military is monitoring the situation in case of additional launches and maintaining a readiness posture", the JCS statement said.
In Washington, a State Department official said the U.S. was monitoring the situation and consulting with allies in the region. "We will increase our vigilance to preserve the safety and assets of the Japanese people", he told reporters. That launcher is apparently the same system that South Korea's military said was likely used in Thursday's launches.
"Abe may see what a real ballistic missile is in the not distant future and under his nose".
The Washington Post says Thursday's test was an expression of the North's frustration over the Trump administration's refusal to grant it sanctions relief. South Korea on Tuesday sent a formal protest message to North Korea, saying the drills violated last year's inter-Korean agreements aimed at reducing front-line military animosities.
It's the second time that South Korea has fired warning shots to drive back a North Korean ship in the area since South Korea's current liberal government took office in 2017. North Korea has previously conducted missile tests on important American holidays. President Trump has not commented, but in the past expressed ambivalence about tests of short-range rockets.
The E-8C carries out surveillance missions whenever North Korea tests medium- and long-range ballistic missiles.
North Korea has fired other new weapons in recent months in what some experts say is an attempt to wrest concessions from the United States in nuclear diplomacy.
North Korea's State Affairs Commission, the country's supreme decision-making body, condemned on November 13 the U.S.
The sources said that the projectiles appeared to be ballistic missiles - which Pyongyang is banned from firing under United Nations sanctions.
The testings may be a response to recent military drills that South Korea performed along with the USA military.
The move is widely regarded by analysts as being aimed at piling more pressure on the United States in regard to a year-end deadline Kim has set for the U.S. to show flexibility in the two country's stalled denuclearization talks.
After the working-level meeting, South Korea and Japan agreed to hold the director general-level dialogue during the third week of December, or from Dec. 16-20, in Tokyo, the trade ministry noted.
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