North Korean leader Kim Jong Un commissioned his second naval destroyer and called for building a more reliable nuclear deterrent, state media reported Monday, as the U.S., South Korea and Japan began a trilateral military exercise he views as a security threat.

The development comes as North Korea is dialing up pressure on the U.S. after rebuffing President Donald Trump's decision to scale back a major military drill with South Korea in a conciliatory step toward Kim.

Observers say North Korea wants to wrest bigger U.S. concessions.