General Dan Caine, the top US military officer, held talks with his South Korean counterpart Kim Myung-soo, and Japan's General Yoshihide Yoshida, as part of an annual meeting on regional security.
They discussed growing military ties between Moscow and Pyongyang, which has sent weapons and thousands of troops for Russia's war in Ukraine.
The talks covered "the potential transfer of military technology from Russia to the DPRK", the three military chiefs said in a statement which used North Korea's official name.
They called on the nuclear-armed North to "immediately cease" all such unlawful activities.
Speaking at the start of the trilateral talks, Caine said that North Korea and China are both "undergoing an unprecedented military build up with a clear and unambiguous intent to move forward with their own agendas".
"We need to be mindful of that, we need to be able to demonstrate resolve, to be entrepreneurial and proactive in our partnerships," he added.
Separately, the three countries conducted joint air drills on Friday over South Korea's southern island of Jeju, involving a US B-52H strategic bomber, according to the defence ministry.
Washington, Seoul's long-time security ally, stations around 28,500 troops in the South.
The two countries signed a new five-year agreement in 2024 on sharing the cost of those troops, but this week Trump said South Korea is "paying very little for the military".
Trump, who has also threatened Seoul with 25 percent tariffs unless they can negotiate a trade deal, said South Korea "should be paying for their military".
South Korea's foreign ministry said this week that they were "committed to faithfully implementing" the defence cost deal, "which was validly concluded and has entered into force".
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