North Korea had launched two missiles last Wednesday as well (File Photo)

North Korea fired two short-range ballistic missiles early Wednesday morning, South Korea's Joint Chiefs of Staff said in a statement.

The missile launch was the second in a week, after Pyongyang also fired two short-range ballistic missiles into the Sea of Japan on Thursday.

North Korea said last week's launch was a direct response to planned U.S.-South Korea joint military exercises and the South's deployment of what it called "ultramodern offensive weapons," almost certainly a reference to its import of U.S. F-35A stealth fighters.

Although a ballistic missile test is a violation of United Nations Security Council resolutions, and the missiles are designed to threaten South Korea, President Donald Trump played down the significance of last week's test, saying many countries test short-range missiles.

North Korea has also threatened to pull out of denuclearization talks with the United States if the military exercises go ahead, saying they would break a promise made by Trump to Kim Jong Un when the two leaders met at the demilitarized zone (DMZ) between the two Koreas in June.

The launches have helped dampen optimism that might have arisen out of that meeting, and they underlined the challenges ahead.

Secretary of State Mike Pompeo said Tuesday that he did not know when the next working-level talks would begin but he expressed hope that they start "before too long."

"Chairman Kim said when the two leaders met at the DMZ that they would start in a few weeks. It's taking a little bit longer than that," he told reporters on his plane before a refueling stop in Alaska. "I hope . . . before too long we'll have special representative [Stephen] Biegun sitting with what I think will be his new counterpart for North Korea."

South Korea said the missiles had been launched from near the Wonsan-Kalma area, the site of many previous missile launches in the past, on North Korea's east coast, at 5:06 and 5:27 a.m. They flew about 150 miles and reached an altitude of about 20 miles.

"North Korea's continued missile launches do not help relieve tensions on the Korean Peninsula and we urge North Korea to stop such acts," the country's Joint Chiefs of Staff said.

Japan's Defense Ministry said Wednesday's missile launch had not encroached on its waters and did not immediately affect its security.

While the Trump administration has responded calmly to the missile launches, experts said they represent a significant security threat. They also pose serious questions for Trump and South Korean President Moon Jae-in.

"While not surprising, certainly not helpful to the diplomatic process," tweeted Jenny Town, managing editor of the Stimson Center's 38 North website. "How long can Moon maintain his cool before he starts to take this personally? At what point does this sour the Trump administration's political will to return to negotiations? Tough choices ahead."

North Korea said last week's missile launch was a "solemn warning to South Korean military warmongers." It said the "tactical guided missiles" were designed to fly at low altitude with a "leaping flight," to make them hard to detect and intercept.

Leif-Eric Easley, a professor at Ewha University in Seoul, said it was not appropriate to shrug off the tests as "short-range."

"These missiles represent technological developments that threaten U.S. allies and forces in Asia," he said. "These tests also reflect North Korean dissatisfaction with the current terms of diplomatic engagement."

Scott Snyder, a Korea expert at the Council on Foreign Relations, said Trump's subdued response to last week's launches has given Pyongyang no incentive to stop testing.

"President Trump has said he is not bothered by it, so why not?" he said. "North Korea is normalizing the idea that they can continue testing below a certain threshold."