US President Barack Obama has joined with South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe to present a united front over what he called "provocations" committed by North Korea in its recent nuclear and missile tests.
Meeting on the sidelines of a global nuclear security summit in Washington, the three leaders recommitted their countries to each other's defence and warned they could take further steps to counter threats from Pyongyang.
Obama held separate talks with President Xi Jinping of China, the closest North Korea has to an ally, and said they both wanted to see "full implementation" of the latest United Nations sanctions against Pyongyang.
But Xi offered no sign that Beijing was prepared to go beyond its consent to the Security Council measures imposed in early March.
"We are united in our efforts to deter and defend against North Korean provocations," Obama told reporters after the US-Japan-South Korea meeting.
"We have to work together to meet this challenge."
Relations between Park and Abe have been frosty in the past, but the two have been brought together in recent months by shared concerns about North Korea, which conducted a fourth nuclear bomb test on January 6 and launched a long-range rocket into space in February.
The expanded UN sanctions aimed at starving North Korea of funds for its nuclear weapons and ballistic missile programs were approved in a unanimous Security Council vote on a resolution drafted by the United States and China.
But even though Beijing has signed on, doubts persist in the West on how far it will go in tightening the screws on impoverished North Korea, given Chinese concerns about fuelling instability on its borders.
Appearing later with Obama, Xi said that while Washington and Beijing disagree in some areas, they have had "effective communication and co-ordination" on the North Korean issue.
However, China, considered the most capable of influencing North Korea's reclusive leadership, has said repeatedly that sanctions are not the solution and only a resumption of international talks can resolve the dispute with Pyongyang.
Six-party talks between the two Koreas, China, the US, Japan and Russia aimed at curbing the North's nuclear ambitions collapsed after the last round in 2008.
Obama said he, Park and Abe had directed their teams to come up with additional steps they can take collectively against North Korea.
Park said the leaders had discussed ways to force North Korea to "alter its misguided calculus" on its weapons programs, and Abe expressed a commitment to strengthening trilateral security co-operation.