SEOUL, South Korea -- North Korea on Friday fired two medium-range ballistic missiles into the sea, days after its leader Kim Jong Un ordered tests likely aimed at developing technology it needs to acquire to build a missile capable of reaching the U.S. mainland.

South Korea's Joint Chiefs of Staff said in a statement that the missiles flew 500 miles before crashing off the North's east coast on Friday.

South Korean military officials said it wasn't immediately known what type of missiles were fired. But a South Korean defense official, requesting anonymity citing department rules, said it was the first medium-range missiles launched by the North since April 2014 when it fired two.

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U.S. Army Lt. Col. Michelle Baldanza said in a statement, "Launches using ballistic missile technology are a clear violation of multiple U.N. Security Council resolutions. We call on North Korea to refrain from actions that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments and obligations."

State Department spokesman John Kirby said in a statement that the U.S. was "closely monitoring the situation."

"We call again on North Korea to refrain from actions that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments and obligations," Kirby said.

"The United States remains steadfast in its commitments to the defense of its allies, including the Republic of Korea and Japan. We will continue to coordinate closely with our allies and partners in the region," Kirby said.

Friday's launch came amid a heightened international standoff over the North's weapons programs in the wake of its nuclear test and long-range rocket launch earlier this year.

In recent weeks, North Korea threatened preemptive nuclear strikes against Washington and Seoul and test-fired short-range missiles and artillery into the sea in response to tough U.N. sanctions imposed over its nuclear test and rocket launch. The North says it needs nuclear weapons to cope with what it calls U.S. military threats.

On Tuesday, North Korea's state media said Kim had ordered tests of a nuclear warhead and ballistic missiles capable of carrying warheads. Kim issued that order while overseeing a successful simulated test of a re-entry vehicle aimed at returning a nuclear warhead into the atmosphere from space so it could hit its intended target.

This led South Korean analysts to suspect that the North would likely fire a missile soon to test the re-entry technology.

Some analysts had also predicted the North might fire a missile carrying an empty warhead, which contain trigger devices but lack plutonium or uranium, to see if those warhead's parts can survive the high pressure and temperatures upon re-entry into the atmosphere and if they were able to detonate at right time.

Outside experts said it is the last major technology that North Korea must master to achieve its goal of developing a long-range missile capable of hitting the U.S. mainland.

South Korean defense officials said North Korea hadn't yet to acquire the re-entry technology so that it doesn't yet have a functioning intercontinental ballistic missile.