World leaders including US President Barack Obama Monday will launch a summit on the threat from nuclear-armed terrorists, but the atomic ambitions of North Korea and Iran are set to feature heavily.

North Korea's upcoming rocket launch has overshadowed the run-up to the two-day meeting in Seoul, 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 of China, Russia and host South Korea during his visit.

The nuclear-armed North says its rocket will merely put a peaceful satellite into orbit. The United States and 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 that North Korea would 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 (in Seoul), to try to discourage North Korea from going ahead with the proposed satellite launch," he told South Korea's Yonhap news agency on Friday.

Leaders or senior officials from 53 nations will attend the Nuclear Security Summit, with Interpol, the International Atomic Energy Agency (IAEA), the European Union and the UN also taking part.

The IAEA, while worried about nuclear proliferation by North Korea, also suspects that Iran is bent on making nuclear weapons. Iran says its uranium enrichment activities are peaceful.

Neither Iran nor North Korea are on the formal agenda in Seoul.

But leaders of five nations involved in stalled nuclear negotiations with the North -- the United States, South Korea, China, Russia and Japan -- will all be present, offering an opportunity for consultations.

Pyongyang sees the summit as a chance for Washington and Seoul to 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.

Seoul says 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 in Washington in 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 republic Kazakhstan secured over 13 tonnes of highly enriched uranium (HEU) and plutonium, while Chile eliminated its entire HEU stockpile, the report said.

The United States and Russia signed a protocol under which each will dispose of 34 tonnes of plutonium -- enough for 17,000 nuclear weapons.

Russia ended plutonium production. Ukraine eliminated two-thirds of its HEU and was expected to dispose of the rest by the Seoul summit.

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 former Soviet 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 a Seoul forum Thursday.

The summit agenda has been expanded to cover the securing of radioactive material, freely available from hospitals and other sources.

Stanford University expert 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 after Japan's March 2011 Fukushima disaster.

Experts say the accident showed terrorists could create the same conditions as a tsunami did, by damaging cooling systems and cutting off power.