North Korea's upcoming rocket launch has overshadowed the run-up to the two-day meeting inSeoul, which seeks agreement on locking down fissile material that could be used to build thousands of terrorist bombs.
Obama will hold talks on the launch plan and other issues with leaders ofChina,Russiaand hostSouth Koreaduring his visit.
The nuclear-armed North says its rocket will merely put a peaceful satellite into orbit. TheUnited Statesand others believe next month's launch will test a long-range missile which could one day deliver an atomic warhead.
Gary Samore, coordinator for arms control at the US National Security Council, warned thatNorth Koreawould face a "strong response" from Washington and its allies if it goes ahead with the launch.
"We will be working with other countries, when President Obama is here (inSeoul), to try to discourageNorth Koreafrom going ahead with the proposed satellite launch," he toldSouth Korea's Yonhap news agency on Friday.
Leaders or senior officials from 53 nations will attend the Nuclear SecuritySummit, with Interpol, the International Atomic Energy Agency (IAEA), the European Union and the UN also taking part.
The IAEA, while worried about nuclear proliferation byNorth Korea, also suspects thatIranis bent on making nuclear weapons.Iransays its uranium enrichment activities are peaceful.
NeitherIrannorNorth Koreaare on the formal agenda inSeoul.
But leaders of five nations involved in stalled nuclear negotiations with the North -- theUnited States,South Korea,China,RussiaandJapan-- will all be present, offering an opportunity for consultations.
Pyongyangsees the summit as a chance forWashingtonandSeoulto gang up on it. Any South Korean move to address the North's nuclear programme at the summit would be seen as a declaration of war, it has vowed.
Seoulsays the formal event is not about nations but "non-state actors".
Obama in a 2009 speech described nuclear terrorism as "the most immediate and extreme threat to global security".
He announced a drive to secure all vulnerable nuclear material worldwide within four years, a process which led to the first nuclear security summit inWashingtonin April 2010.
A joint report by the Washington-based Arms Control Association (ACA) and the Partnership for Global Security (PGS), which campaign against nuclear proliferation, acknowledged major progress since then.
Former Soviet republicKazakhstansecured over 13 tonnes of highly enriched uranium (HEU) and plutonium, whileChileeliminated its entire HEU stockpile, the report said.
TheUnited StatesandRussiasigned a protocol under which each will dispose of 34 tonnes of plutonium -- enough for 17,000 nuclear weapons.
Russiaended plutonium production.Ukraineeliminated two-thirds of its HEU and was expected to dispose of the rest by theSeoulsummit.
But experts say much more must be done to end an apocalyptic threat.
"The commitments on the books will not get the job done," said Michelle Cann of PGS, the report co-author.
"To prevent nuclear terrorism in the years ahead, the global nuclear security system must grow and adapt to new threats," she said.
"There is a danger that early successes of the summit process will lead to complacency."
The ACA says there have been 16 confirmed cases of unauthorised possession of HEU or plutonium documented by the IAEA since 1993, mainly in the formerSoviet Union.
Alexandra Toma of the Connect US Fund, which promotes nuclear non-proliferation, said a sophisticated extremist group could plausibly take advantage of such lapses.
"It takes only 50 kilograms (110 pounds) of highly enriched uranium to make a crude nuclear bomb" the size of a grapefruit, she told aSeoulforum Thursday.
The summit agenda has been expanded to cover the securing of radioactive material, freely available from hospitals and other sources.
StanfordUniversityexpert Siegfried Hecker told the Thursday forum the most likely nuclear threat was a "dirty bomb... a weapon of mass disruption" since radiation sources were everywhere.
The meeting will also discuss the link between nuclear security and nuclear safety afterJapan's March 2011Fukushimadisaster.
Experts say the accident showed terrorists could create the same conditions as a tsunami did, by damaging cooling systems and cutting off power.
Copyright AFP (Agence France-Presse), 2012