The United States moved Thursday to keep pressure on nuclear-armed North Korea and reassure China over the proposed deployment of a sophisticated anti-missile system to South Korea.
After meeting with South Korean and Japanese allies at the start of a nuclear summit in Washington, President Barack Obama said there is a need to "vigilantly enforce the strong UN security measures" against Pyongyang, following provocative nuclear and missile tests that violated several UN resolutions.
North Korea in January detonated a nuclear device and launched a long-range rocket a month later, the latest in a series of tests that have alarmed regional neighbors.
The White House wants to keep up pressure on the North Korean regime, increasing the economic and diplomatic cost of ignoring international appeals to mothball its nukes.
"The international community must remain united in the face of North Korea's continued provocations," Obama wrote in an article published on the eve of the summit.
"The additional sanctions recently imposed on Pyongyang by the United Nations Security Council show that violations have consequences."
Obama kicked off the nuclear security summit by holding discussions with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-Hye, followed by a separate sit-down with President Xi Jinping of China.
The United States and South Korea have begun discussions on deployment of THAAD -- the Theater High Altitude Area Defense System, a sophisticated missile system.
US delegates want to reassure Beijing that such a system poses no threat to China, which fears it will deepen US influence in the region.
"It is designed and capable only of responding to the North Korean threat," said Dan Kritenbrink, senior Asian affairs director at the National Security Council.
"It in no way threatens either Chinese or Russian or other security interests in the region and will do nothing to undermine strategic stability between the United States and China."
Obama will later meet Xi, his only full bilateral meeting with the dozens of world leaders coming to Washington.
He will also hold what the White House is calling a "brief" meeting with France's President Francois Hollande.
China is seen as key to making sanctions against North Korea bite and believes China could more forcefully wield its influence over Pyongyang, including encouraging its Stalinist neighbor to tone down destabilizing rhetoric.
Tensions are only expected to rise in advance of a major Communist Party Congress in North Korea in May.
And Obama and Xi are also likely to touch on disputes in the South China Sea, where Beijing has occupied and is building on contested islands.
Washington fears the islets and atolls may be used for military purposes.
US officials see a steady series of meetings and exchanges between Obama and Xi as vital in limiting disagreements.