North Korea fired two ballistic missiles into the Sea of Japan Friday, a U.S. defense official told Fox News.

Both missiles are believed to be Nodong medium-range ballistic missiles launched from a road-mobile launcher, according to the official.

"Neither was assessed to be a threat to the U.S. or our regional allies," the official told Fox News.

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

The launch came days after North Korean leader Kim Jong Un ordered tests of a nuclear warhead and ballistic missiles capable of carrying warheads.

North Korea had also said it succeeded in a simulated test of a re-entry vehicle aimed at returning a nuclear warhead safely back to the atmosphere from space during a missile launch.

State Department spokesman John Kirby said the agency is "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," he said.

Taken together, Seoul analysts said Pyongyang would likely fire a missile to test the re-entry technology that it needs acquire to develop a long-range nuclear missile.

Some analysts had also predicted the North might install on a dummy device on a missile or even empty warheads, 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.

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 pre-emptive 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.

Fox News' Jennifer Griffin, Lucas Tomlinson and the Associated Press contributed to this report.