The United States' top military officer said on Thursday that there is a "real sense of urgency" about building up three-way defence ties with South Korea and Japan to deter North Korea.
Admiral Mike Mullen said Pyongyang's "reckless behaviour, enabled by their friends in China" had made north-east Asia a volatile place.
The United States has conducted separate military manoeuvres with the two East Asian allies since North Korea ramped up regional tensions with its deadly November 23 artillery attack on a South Korean border island.
Mullen said in Seoul on Wednesday that he had high hopes for pacifist Japan's participation in US-South Korean drills, to promote regional security after North Korea's bombardment killed four people on the island.
Asked about the comment in Tokyo on Thursday, he said: "I do have a real sense of urgency about addressing the potential in terms of the Korean peninsula that is much better addressed with all of us together, in terms of showing strength and getting to a point where we can deter North Korean behaviour." "That benefits all of us," added Mullen, chairman of the Joint Chiefs of Staff, after talks with Japan's Defence Minister Toshimi Kitazawa.
"North-east Asia is today more volatile than it's been in much of the last 50 years. Much of that volatility is owed to the reckless behaviour of the North Korean regime, enabled by their friends in China," he said.
China is the last major ally of communist North Korea and its lifeline for food and energy aid. Beijing has resisted US calls to pressure Pyongyang to change its ways, instead urging talks with the regime.
The United States and Japan have been key security allies since the end of World War II. Japan hosts US bases while its own Self Defence Forces are constrained by a post-war pacifist constitution.
Debate about changing Japan's defence posture, which would require constitutional amendments, is highly controversial in the country and viewed with suspicion by many of Japan's Asian neighbours.
Mullen said, "I think all leaders, civilians and military, need to figure out a way in this world, in this region in the world, to work together and to be less tied to our past, which sometimes can hold back initiatives for the future, which is really important as we look at the world."
In his earlier talks with Kitazawa, Mullen also said, "it was very important for Japan to engage in tripartite cooperation with the United States and South Korea," according to a Japanese defence ministry official.