The top U.S. naval officer said Tuesday that forces under his command are "fully prepared" for any further nuclear or missile tests undertaken by North Korea, hinting that further pressure, including from the military, can be applied if needed.

"We are more challenging (to North Korea) if we are less predictable," Adm. John Richardson, chief of U.S. naval operations, said in an interview with Kyodo News and other media at the navy's Yokosuka base near Tokyo.

In the face of growing threats posed by North Korea's nuclear development and ballistic missile launches, the U.S. Navy and the Japanese Maritime Self-Defense Force carried out a joint exercise in the Sea of Japan in November. The drill saw three navy aircraft carriers and three MSDF destroyers deployed.

Richardson said, however, the exercise had "no provocative dimension" as it was a programmed drill.

Earlier in the day, the admiral expressed concern about the current situation surrounding North Korea, calling it an "unprecedented and urgent threat," as he spoke to some 1,500 crew of the U.S. aircraft carrier Ronald Reagan at the Yokosuka base.

Richardson underlined the need for keeping "firm and set military options" in order to promote diplomatic resolutions.