Nuclear summit to begin under N. Korean shadow

US President Barack Obama and dozens of other world leaders will begin a summit Monday on curbing the threat of nuclear terrorism, but North Korea\'s atomic plans will be in focus on the sidelines. The two-day meeting in South Korea is a follow-up to an inaugural summit in Washington in 2010 hosted by Obama, which kick-started efforts to lock up fissile material around the globe that could make thousands of bombs. Obama announced on the eve of the Seoul event, which will gather leaders or top officials from 53 nations, that Ukraine had fulfilled a pledge made two years ago to remove all highly enriched uranium from its territory. \"I believe it is a preview of the kind of progress we are going to see over the next two days in confronting one of the most urgent challenges to global security -- the security of the world\'s nuclear weapons and preventing nuclear terrorism,\" Obama said. While North Korea\'s nuclear programme is not officially on the agenda in Seoul, it is expected to be intensely discussed on the sidelines as world leaders take advantage of the opportunity of face-to-face meetings. Tensions have escalated in recent weeks after North Korea announced it would launch a long-range rocket in April. The nuclear-armed North says its rocket will merely put a peaceful satellite into orbit. But the United States and many other countries believe the launch is intended to test a long-range missile that could one day deliver an atomic warhead. Obama and South Korean President Lee Myung-Bak presented a united front against North Korea during a press conference on Sunday, warning it again against the rocket launch and further \"provocative\" actions. \"North Korea will achieve nothing by threats or by provocations,\" Obama said. Lee added: \"President Obama and I have agreed to respond sternly to any provocations and threats by the North and to continually enhance the firm South Korea-US defence readiness.\" Obama also sought to step up pressure on China, North Korea\'s chief international ally, which has declined to speak out strongly against Pyongyang in relation to the planned rocket launch. \"My suggestion to China is that how they communicate their concerns to North Korea should probably reflect the fact that the approach they have taken over the last several decades has not led to a fundamental shift in North Korea\'s behaviour,\" Obama said. Obama is scheduled to meet Chinese President Hu Jintao on Monday, and separately with Russian President Dmitry Medvedev. The United States, China and Russia, along with Japan and South Korea, are involved in long-running and currently stalled negotiations with the North aimed at convincing it to give up its atomic ambitions. Iran\'s nuclear ambitions are similarly not on the agenda in Seoul but the leaders of the world powers may take the opportunity of their face-to-face meetings to discuss US-led efforts to curtail Tehran\'s programme. Experts have acknowledged major progress on the fissile material front since the Washington summit. They point to former Soviet republic Kazakhstan securing over 13 tonnes of highly enriched uranium (HEU) and plutonium since then, while Chile eliminated its entire HEU stockpile. The United States and Russia also signed a protocol under which each will dispose of 34 tonnes of plutonium -- enough for 17,000 nuclear weapons. But experts say much more must be done to end an apocalyptic threat.