His measured response this time suggests he takes the SLBM launch more seriously.

This week's launch was seen as a move aimed at increasing the North's leverage ahead of the high-stakes talks with the US as the firing came a day after Pyongyang announced that it had agreed with the US to resume working-level discussions on Saturday following preliminary contact on Friday.

Along with its intercontinental ballistic missiles, the North's SLBM program is considered one of the biggest threats to the US and its allies, as it could extend the range of the North's nuclear missiles. SLBMs are also hard to detect before they emerge from the water.

US Defense Secretary Mark Esper agreed in a phone call Thursday with his Japanese counterpart that such 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 a press briefing.

North Korea's chief nuclear negotiator, Kim Myong-gil, arrived at a Stockholm airport via Beijing Thursday and appeared to head straight to the North Korean Embassy.

Earlier in Beijing, he struck an optimistic tone.

"(We) are heading to working-level negotiations with the US," he told reporters at Beijing International Capital Airport. "As the US side sent a new signal, I bear high expectations and optimism, and I am also optimistic about the results."

The two sides have not had formal negotiations since the second summit between Trump and Kim in February ended without any agreement.

The new talks 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.

Trump suggested last month that a "new method" could be good to move the negotiations forward.

If the talks produce progress, a third summit between Trump and Kim is widely expected to happen before the end of the year. (Yonhap)