The US Joint Chiefs of Staff said Thursday that North Korea's latest test was of a short to medium range ballistic missile, which was fired from a sea-based platform.

JCS spokesman Air Force Col. Patrick Ryder told reporters there was no indication the missile on Wednesday was launched from a submarine, but declined to provide other details.

The comments come after North Korea said it successfully tested a new submarine-launched ballistic missile off its east coast, "usher(ing) in a new phase in containing the outside forces' threat to the (North) and further bolstering its military muscle for self-defense."

"As we understand it, North Korea fired a short to medium range ballistic missile some 280 miles (450 kilometers) into the" East Sea, Ryder said at a press briefing. "And what we know is that the missile was fired from a sea-based platform."

South Korea's military said earlier that the North appeared to have fired an SLBM that flew around 450 km at a maximum altitude of about 910 km.

Experts say the missile could have flown a greater distance had it been fired at a normal angle, not "in vertical mode."

The launch raised tensions again ahead of the resumption of working-level denuclearization negotiations between the North and the US in Sweden this weekend.

Submarine-launched missiles are harder to detect than ground-based ones, increasing the threat posed to the US and its allies.

US President Donald Trump still maintained a cautious tone.

"We'll see," he told reporters Thursday when asked if the North had gone too far this time. "They want to talk, and we'll be talking to them soon. We'll see."

Trump has played down the North's previous tests of short-range projectiles and ballistic missiles this year as "very standard," saying they may be in breach of UN Security Council resolutions but they are not a violation of North Korean leader Kim Jong-un's promise to him to stop nuclear weapons and long-range missile tests.

North Korea's chief negotiator, Kim Myong-gil, arrived in Stockholm Thursday after telling reporters on a stop in Beijing that he was optimistic about the upcoming talks.

"As the US side sent a new signal, I bear high expectations and optimism," he said without elaborating.

The two sides are expected to focus on finding common ground between US demands for the North's complete and verified denuclearization and Pyongyang's demands for sanctions relief and security guarantees.

US Defense Secretary Mark Esper and Japanese Defense Minister Taro Kono held a phone call Thursday and discussed the North Korean launch.

Esper and Kono "agreed that the North Korea tests are unnecessarily provocative and do not set the stage for diplomacy and that North Korea should cease these tests," Pentagon spokesman Jonathan Hoffman said at the briefing alongside Ryder. (Yonhap)