[WASHINGTON] The United States turned up the heat on North Korea's nuclear program Thursday as world leaders gathered in Washington to reduce radioactive stockpiles and protect remaining inventories from terrorists.
The threat of militants getting their hands on nuclear material has gained fresh urgency since last week's attacks in Brussels, where it later emerged that two bombers had links to surveillance tape of a top official from a Belgian nuclear facility.
The latest Nuclear Security Summit saw President Barack Obama invite about 50 world leaders to Washington for the fourth and final meeting of its kind under his presidency.
Aside from pushing for better nuclear material safeguards, Mr Obama wants to pressure nuclear-armed North Korea.
After meeting with South Korean and Japanese allies, Mr Obama said there is a need to "vigilantly enforce the strong UN security measures" against Pyongyang.
In January, North Korea detonated a nuclear device and a month later launched a long-range rocket, the latest in a series of banned tests.
The White House wants to increase the economic and diplomatic cost to North Korea of ignoring international appeals to mothball its nukes.
"It is important to the entire international community to vigilantly enforce the strong UN security measures that were passed in light of some of the ballistic missile and nuclear activity that Pyongyang has been engaging in," Mr Obama said.
The United States and South Korea have begun discussions on deployment of THAAD - the Theater High Altitude Area Defense System, a sophisticated missile system.
This has raised concerns in Beijing, which is unhappy at the prospect that US missiles on its doorstep will further tip the balance of power in the Pacific towards Washington.
"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," insisted Dan Kritenbrink, a top Obama adviser.
Mr Obama also met with President Xi Jinping of China, opening the meeting with a promise of "candid" discussion over Beijing's suspected military buildup in the South China Sea.
US officials worry China's actions are not consistent with Mr Xi's pledge at the White House last year not to pursue militarisation of the hotly contested and strategically vital waterway.
China is seen as key to making sanctions against North Korea bite. The United States believes Beijing could more forcefully wield its influence over Pyongyang, including encouraging its Stalinist neighbor to tone down destabilizing rhetoric.
Ahead of the meeting, the White House announced that the United States and China would be among the countries signing the Paris climate accord on April 22 in New York.
Mr Obama and Mr Xi are expected to meet again at the G20 in China in September.
The specter of the Islamic State group obtaining a "dirty bomb" also loomed over the summit, and the meeting comes just days after 32 people were killed and 340 were injured in bombings at Brussels airport and the Belgian capital's metro.
The attacks featured conventional explosives, but two of the suicide bombers - Ibrahim and Khalid El Bakraoui - have been linked to 10 hours of video surveillance detailing the comings and goings of a senior Belgian nuclear official.
Few believe the IS group could develop a nuclear weapon, but many fear it could acquire uranium or plutonium and construct a "dirty bomb." Such a device would not trigger a nuclear explosion but would scatter radioactive material - with potentially devastating physiological, medical and economic effects.
Nuclear material can be found in small quantities at universities, hospitals and other facilities the world over, often not well secured.
Since the mid-1990s, almost 2,800 incidents of illicit trafficking, "unauthorised possession" or loss of nuclear materials have been recorded in an International Atomic Energy Agency database.
More than 50 heads of state were invited to the summit, but the absence of leaders from Russia, North Korea, Iran and Belarus virtually ensures gaps in the united front.
White House spokesman Ben Rhodes said Russia's decision not to attend at the highest level was a missed opportunity for Moscow, which itself faces significant threats of its own.
"Russia's lack of participation ... is, frankly, counterproductive given this is an area where we share an interest," he said. "We want Russia at the table on issues of nuclear security." Though he was not at the summit, Donald Trump also drew attention after the Republican presidential frontrunner earlier suggested Asian allies should develop atomic weapons.
Mr Trump's remarks that he would pull troops from South Korea and Japan and allow those countries to develop nukes drew a scathing rebuke from Deputy National Security Advisor Ben Rhodes.
"The entire premise of American foreign policy as it relates to nuclear weapons for the last 70 years has been focused on preventing the proliferation of nuclear weapons," said Mr Rhodes, one of Mr Obama's closest aides.
Attention was also diverted by Turkey's President Recep Tayyip Erdogan, who was greeted at a separate Washington event by noisy protesters, and whose body guards attacked members of the media.