President Barack Obama Sunday warned that nuclear-armed terrorists would pose the single biggest threat to the United States, before a global summit aimed at thwarting such a nightmare scenario.

Obama sought support from fellow leaders for his effort to secure all loose nuclear material around the world within four years, on the eve of a 47-nation conference -- the largest summit chaired by a US president in 65 years.

He conjured up the horrific possibility of a nuclear detonation in New York City, London or Johannesburg, and the serious economic, political and security trauma that would result, to characterize the gravity of the threat.

"The single biggest threat to US security, both short-term, medium-term and long-term, would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said.

"This is something that could change the security landscape of this country and around the world for years to come."

"We know that organizations like Al-Qaeda are in the process of trying to secure a nuclear weapon -- a weapon of mass destruction that they have no compunction at using."

Despite the focus on extremist groups, two states, however, Iran and North Korea, which already has the bomb, will cast a shadow over the two-day summit which opens on Monday.

Washington is leading an effort to toughen sanctions within weeks on Iran over its nuclear program, which the United States and allies say is aimed at producing weapons, a charge Tehran denies.

The White House will seek concrete commitments from world leaders on securing stockpiles of separated plutonium and uranium, to ensure that they cannot be stolen, smuggled or sold to extremists.

"The threat of nuclear war... has diminished. The threat of nuclear terrorism has increased," US Secretary of State Hillary Clinton told ABC News.

To kick off his counter-proliferation drive, Obama met Kazakhstan's President Nursultan Nazerbayev and South African President Jacob Zuma -- praising both for handing over or disarming Cold War-era nuclear weapons.

He also held talks with prime ministers Manmohan Singh and Yousuf Raza Gilani of nuclear-armed rivals India and Pakistan.

Obama, who last week signed a landmark disarmament treaty with Russia and laid out a new US nuclear strategy limiting how Washington could use atomic weapons, said he was confident that the summit would garner important progress.

"I feel very good at this stage in the degree of commitment and sense of urgency that I've seen from the world leaders so far on this issue," Obama said.

"We think we can make enormous progress on this."

The summit itself will focus primarily on separated plutonium and highly enriched uranium stocks, rather than radiological "dirty" bombs, which US officials see as a less catastrophic threat than nuclear devices.

US officials hope nations at the summit will agree a series of their own security steps for their own nuclear material, and help pay to put the stocks of less well-off countries under lock and key.

They also expect some leaders to unveil specific actions, similar to Chile's decision to ship a stock of highly enriched uranium to the United States.

The conference is also a precursor to the United Nations Non-Proliferation Treaty review conference next month, seen as another important moment in heading off a future nuclear arms race.

As well as presiding over the summit, Obama will Monday meet Chinese President Hu Jintao in talks likely to focus partly on US hopes that China will let its yuan currency find a market level.

The White House also announced that the US leader would have a bilateral meeting with Turkish Prime Minister Recep Tayyip Erdogan.

The talks will occur just over a week after Turkey decided to return its ambassador to Washington after a row over moves in Congress to brand the World War I massacres of Armenians as genocide.

Turkey is also seeking to revive stalled reconciliation efforts with Armenia. Obama may play a part in that effort, when he meets Armenian President Serzh Sarkisian in a separate bilateral meeting on Monday.

But some key world leaders will not be at the summit, including Israeli Prime Minister Benjamin Netanyahu, who has a tense relationship with Obama and reportedly feared Muslim states would raise Israel's undeclared nuclear status.

The British and Australia prime ministers also sent their excuses.

earlier related report
Obama to open unprecedented nuclear summit Monday
Washington (AFP) April 10, 2010 - US President Barack Obama will call for unprecedented global action to secure nuclear stocks and keep weapons grade material out of the hands of extremists, at a two-day summit opening Monday.

The White House says Obama called the 47-nation conference to put together the most coordinated effort yet to tackle what one adviser called the most dangerous security threat haunting America, and the rest of the world.

"It is absolutely fundamental to view this summit with the starting point of the grave nature of the threat of nuclear terrorism," said Ben Rhodes, a deputy US national security advisor.

"We know that terrorist groups, including Al-Qaeda, are pursuing the materials to build a nuclear weapon, and we know that they have the intent to use one."

The nuclear security summit -- the biggest gathering of world leaders led by a US president since 1945 -- marks Obama's boldest effort yet to exercise global leadership on one of his principal foreign policy themes, non-proliferation.

Obama wants fellow leaders to agree to his timeframe of securing all nuclear materials within four years and a final summit communique will likely include calls for tougher prosecutions of traffickers in weapons grade materials.

He enters the meeting, an attempt to halt the theft, sale or smuggling of unsecured nuclear materials, with his political standing enhanced by a new disarmament deal with Russia and by enacting historic domestic health care reform.

Obama will also flex diplomatic muscle on the sidelines of the summit, with a flurry of one-on-one meetings with other leaders, including Chinese President Hu Jintao and South African President Jacob Zuma.

The White House also announced Saturday that Obama would meet his Ukrainian counterpart Viktor Yanukovych for the first time on Monday.

Some key world leaders will not be at the summit however, including Israeli Prime Minister Benjamin Netanyahu, who has a tense relationship with Obama and reportedly feared Muslim states would raise Israel's undeclared nuclear status.

British Prime Minister Gordon Brown, fighting for his job ahead of an election next month also sent his excuses, as did Australian Prime Minister Kevin Rudd who is embroiled in his own health care push.

The summit itself will focus primarily on separated plutonium and highly enriched uranium stocks, rather than radiological "dirty" bombs, which US officials see as a threat, but less potentially catastrophic than nuclear devices.

While the summit is designed at keeping nuclear material out of the hands of extremist groups -- two states not at the summit, Iran and North Korea, will cast a shadow due to the confrontations over their nuclear programs.

Washington is leading an effort to frame a new and biting set of sanctions on Iran, and will seek to lock in Hu's cooperation, after months trying to bring China and Russia on board.

The meeting is the fruition of a long-term effort by Obama to deprive terrorist groups of the means to build nuclear weapons.

Upon becoming a senator in 2005, Obama joined Republican Senator Richard Lugar's efforts to secure global nuclear stocks, and on an early foreign trip as president he vowed to work to eliminate nuclear weapons entirely.

US officials are expecting the nations to agree a series of steps to secure their own material and help pay to put the stocks of less well-off countries under lock and key.

They also expect some leaders to unveil specific actions, similar to Chile's decision to ship a stock of highly enriched uranium to the United States.

The summit will follow the Obama's announcement of a new US nuclear strategy -- which imposed limits on the possible use of the US nuclear arsenal, and after Thursday's signing of a new disarmament treaty with Russia in Prague.

The conference is also a precursor to the United Nations Non-Proliferation Treaty review conference next month.

Non-nuclear states who are signed up to that treaty typically complain that nuclear powers are not living up to their end of the bargain, by taking sufficient steps toward disarmament.

So the new US Strategic Arms Reduction Treaty (START) with Russia may be seen as an attempt to meet those concerns, and jump-start the review conference.