SEOUL, South Korea >> North Korea confirmed Thursday it has carried out its first underwater-launched ballistic missile test in three years, in an apparent bid to dial up pressure on the United States ahead of a weekend resumption of their nuclear diplomacy.

Wednesday’s test of the Pukguksong-3 missile, which North Korea describes as a submarine-launched ballistic missile, is seen as the North’s most high-profile weapons launch since it began diplomacy with the United States early last year. Some experts say North Korea wants to show to the U.S. what would happen if diplomacy fails again.

The Korean Central News Agency said the missile test in the waters off its east coast was successful and “ushered in a new phase in containing the outside forces’ threat to (North Korea) and further bolstering its military muscle for self-defense.”

It didn’t say which outside forces threaten its security. But North Korea has previously said it was forced to develop nuclear-armed missiles to cope with U.S. military threats.

The KCNA report didn’t elaborate on whether the missile was fired from a submarine, a barge or other underwater launch platform. North Korea-dispatched photos showed the missile rising and spewing bright flames above a cloud of smoke from the sea, but the launch platform was not identifiable.

Kim Dong-yub, an analyst from Seoul’s Institute for Far Eastern Studies, wrote on Facebook that the missile was likely fired from a barge built for an underwater launch. He said the missile is under development and that North Korea must test-fire it from a submarine before deploying it.

South Korea’s Defense Ministry said the North Korean missile traveled about 450 kilometers (280 miles) at a maximum altitude of 910 kilometers (565 miles). Ministry officials said the missile flew higher than any other short-range weapons North Korea test-fired in recent months.

KCNA said the missile was launched in a vertical mode and that its test had no adverse impact on the security of neighboring countries. It said Kim sent “warm congratulations” to the national defense scientific research units involved in the test-firing

If fired at a standard trajectory, Kim, the analyst, said the missile could have travelled 1,500-2,000 kilometers (930-1,240 miles). He said the Pukguksong-3 is a medium-range missile. North Korea hadn’t tested a medium- and long-range missiles since its entrance to talks with the United States.

North Korean and U.S. officials are to meet on Saturday to restart diplomacy on how to end the North Korean nuclear crisis. That diplomacy largely remains stalemated after the February breakdown of a second summit between President Donald Trump and North Korean leader Kim Jong Un in Vietnam.

North Korea has recently warned its dealings with the United States may end if Washington fails to come up with new proposals to salvage the nuclear diplomacy by December.

Pukguksong, or Polaris, is a solid-fuel missile in the North’s weapons arsenal. The country first test-launched a Pukguksong-1 missile from an underwater platform in 2016, and Kim said at the time his military had gained “perfect nuclear-attack capability.” A year later, the North test-launched a Pukguksong-2, a land-based variant of the missile.

North Korea having an ability to fire a missile from a submarine is a threat to the United States and its allies because such launches are harder to detect early enough to respond. The use of solid fuel also increases a weapon’s mobility.

After Wednesday’s launch, the U.S. State Department called on North Korea “to refrain from provocations, abide by their obligations under U.N. Security Council resolutions, and remain engaged in substantive and sustained negotiations to do their part to ensure peace and stability on the Korean Peninsula and achieve denuclearization.”

Japan lodged an immediate protest, saying the missile landed inside its exclusive economic zone for the first time since November 2017.