By Kiyoshi Takenaka and Jack Kim
SEOUL (Reuters) - Japan steered off the agenda at a nuclear security summit on Tuesday to hit out at North Korea's plans for a rocket launch next month, as U.S. President Barack Obama cautioned against complacency in dealing with the threat of nuclear terrorism.
The summit was briefly interrupted by a dispute between Argentina and Britain, which went to war in 1982 over the Falkland Islands, over suggestions Britain had sent a submarine capable of carrying nuclear weapons to the South Atlantic.
A communiqué issued at the end of the two-day meeting of more than 50 world leaders in Seoul was light on specifics on how to reduce the risk of atomic materials falling into bad hands, loosely calling for all vulnerable material to be secured in four years.
The world's biggest nuclear concerns, those surrounding the weapons programs of North Korea and Iran, were not on the agenda at the summit, and neither country was invited.
The secretive North has been widely criticized on the sidelines of the meeting, including by main ally China, but host South Korea has explicitly stated the North's weapons of mass destruction programs were off the table during the summit itself.
The forum is meant to deal only with safeguarding nuclear material and facilities and preventing trafficking.
Japanese Prime Minister Yoshihiko Noda ignored protocol and urged the international community to strongly demand North Korea exercise self-restraint over next month's planned rocket launch.
"The planned missile launch North Korea recently announced would go against the international community's nuclear non-proliferation effort and violate U.N. Security Council resolutions," Noda said in an opening speech.
No other major leaders mentioned North Korea's nuclear ambitions or the ballistic missile launch which Pyongyang says will carry a weather satellite into orbit. The West says the launch is a disguised test of a long-range missile designed to reach the American mainland.
North Korea said last week it would consider it a "provocation" if its "nuclear issue is placed on the agenda at the Seoul summit" and if any statement was issued against the North for pursuing such a program.
On Tuesday, it said there was no reason to fire a missile after February's agreement to suspend nuclear and missile tests in return for food aid with the United States.
Obama has said the destitute North could be hit with tighter sanctions if it goes ahead with the rocket launch, but experts doubt China will back another U.N. Security Council resolution against it.
A row erupted during the main session of the summit when British Deputy Prime Minister Nick Clegg hit back at accusations leveled by Argentinean Foreign Minister Hector Timermanan that an "extra-regional power" had sent a submarine capable of carrying nuclear weapons to the South Atlantic.
In front of the world's leaders, Clegg fired back his own missive, calling the remarks "unfounded, baseless insinuations".
Tension between Britain and Argentina is rising as the 30th anniversary approaches of Argentina's invasion of the Falklands that was repulsed by a British task force after a 10-week conflict that killed 650 Argentine and 255 British troops.
Obama told leaders the world was safer because of the steps taken to improve nuclear security, but warned that the threat of the wrong people getting hold of the materials to make a crude atomic bomb was real.
"Nuclear terrorism is one of the most urgent and serious threats to global security," he said.
The communiqué issued at the end of the summit reaffirmed states' commitment to minimizing stockpiles of highly enriched uranium and plutonium, safeguarding nuclear facilities, and preventing illicit trafficking of nuclear and radioactive material.
The long and vaguely worded document, however, offered nothing in the shape of measurable targets and did not single out any state for criticism.
Critics say the summit is no more than a talking-shop, and warn that even though its mandate was extended to include safety after the Fukushima crisis in Japan last year, the next summit in the Netherlands could be the last.
Miles Pomper of the Washington-based Center for Nonproliferation Studies said the Seoul agenda was "underwhelming to say the least".
"You got a lot of juice out of the process the first time because it was a new thing and Obama had just come off the Prague speech," he said, referring to a 2009 address when he declared it was time to seek "a world without nuclear weapons".
"There were a lot of things already in the pipeline, but now we're losing momentum ... we (need to) start being more ambitious."
But heralding the progress made in two years since the first such gathering of world leaders, which he hosted in Washington, Obama said the "security of the world" depended on success.
"It would not take much - just a handful or so of these materials - to kill hundreds of thousands of innocent people. And that's not an exaggeration. That's the reality that we face."
Former Cold War adversaries have cooperated to lock down weapons-grade uranium and plutonium, some countries have agreed to remove all such material from their soil and poorer nations have received financial help to secure nuclear facilities.
"We've come a long way in a very short time, and that should encourage us (but) that should not lead us to complacency," said Obama in an appeal for further collaboration.
Noda, representing a country mired in the world's worst nuclear crisis in 25 years, also said that Tokyo has learned from the Fukushima disaster and was reinforcing power supply devices and increasing security measures at its plants.
An earthquake and tsunami last March knocked out external and on-site power supplies at the nuclear power plant, 240 km (150 miles) northeast of Tokyo, causing the failure of cooling systems and triggering fuel meltdowns, radiation leaks and mass evacuations.
(Additional reporting by Matt Spetalnick, Alister Bull, Yoo Choonsik, Alexei Anishchuk; Writing by Jeremy Laurence; Editing by Nick Macfie and Robert Birsel)