World leaders at a summit on nuclear security in Washington have heard dire warnings of the danger of nuclear material falling into the wrong hands.
President Barack Obama, opening the biggest international meeting hosted by the US since 1945, greeted leaders from nearly 50 countries.
Officials said more should be done to prevent theft or smuggling.
Meanwhile France's leader stressed his country could not give up its own nuclear weapons.
The US welcomed a Ukrainian pledge to eliminate its stockpile of highly enriched uranium by 2012.
The two-day summit is taking place without representatives of Iran and North Korea, neither of whom were invited by the US because of the disputes over their nuclear programmes.
Israeli Prime Minister Benjamin Netanyahu dropped plans to attend the summit, reportedly because of concern that Muslim states planned to press for Israel to open its own nuclear facilities to international inspection.
'Proliferators not welcome'
The head of the International Atomic Energy Agency, Ikia Amano, said that nuclear powers needed to do more to protect nuclear materials.
"The problem is that nuclear material and radioactive material are not well protected and member states need to better protect these materials against the theft or smuggling," he told the BBC.
"On average every two days we receive one new information on an incident involving theft or smuggling of nuclear material."
British Foreign Secretary David Miliband said that nuclear nations like Pakistan were vulnerable.
"The message from this summit is that any country can be treated as a normal country on nuclear matters if it behaves like a normal country," he said.
"Proliferators are not welcome in the modern world, nuclear proliferators especially, and I think it's a very clear message to the Iranians and others that there is an international desire to use civilian nuclear power for beneficial purposes, but not to allow it to leech into a military weapons programme that could be so dangerous, especially in a region like the Middle East."
A senior American counter-terrorism expert, John Brennan, warned that al-Qaeda had been seeking material for a nuclear bomb for more than 15 years.
"There have been numerous reports over the past eight or nine years of attempts to obtain various types of purported material," he told reporters.
"We know al-Qaida has been involved a number of times. We know they have been scammed a number of times."
President Obama and his Chinese counterpart Hu Jintao reportedly agreed at pre-summit talks to step up pressure on Iran over its atomic plans.
"The Chinese very clearly share our concern about the Iranian nuclear programme," said Jeff Bader, Mr Obama's senior director for Asia on the National Security Council.
"The two presidents agreed the two delegations should work together on a sanctions resolution in New York."
Just before the summit opened, Ukraine agreed to eliminate its stockpile of weapons-grade nuclear material which, the US said, was enough to build "several weapons".
White House spokesperson Robert Gibbs: "Ukraine announced a landmark decision"
US officials said Ukraine's highly enriched uranium would be removed with some US technical and financial help.
Ukraine's agreement sets a precedent that Mr Obama would like other countries to follow, the BBC's diplomatic correspondent, Jonathan Marcus, reports from Washington.
It is estimated there are about 1,600 tonnes of highly enriched uranium in the world - the type used in nuclear weapons.
Experts agree that virtually all of it is held by the acknowledged nuclear-weapons states, most of it in Russia.
Speaking in an interview before the summit opened, French President Nicolas Sarkozy said his country would not give up its nuclear weapons because to do so would be to jeopardise national security.
"I cannot jeopardise the security and safety of my country," he told CBS News.
"I have inherited the legacy of the efforts made by my predecessors to build up France as a nuclear power and I could not give up nuclear weapons if I wasn't sure the world was a stable and safe place."
Last week, the US and Russia signed a new Strategic Arms Reduction Treaty, reducing each country's deployed nuclear arsenal to 1,550 weapons.
Mr Obama has also approved a new nuclear policy for the US, saying he plans to cut the nuclear arsenal, refrain from nuclear tests and not use nuclear weapons against countries that do not have them.
Australia has around 25% of the world's reserves of thorium, and a significant amount of the world's reserves of uranium, 40% of the world's high grade uranium. & 90% of the world's low grade uranium.
Australia's PM won't be at world nuclear summit
12th April, 2010.
The United States and Russia have signed an agreement to reduce their nuclear weapons stockpile. It's being described as historic, not just for the relationship between the former enemies but for world security.The signing follows the announcement earlier this week of a more restrained doctrine on the use of U-S nuclear weapons and next week, Washington will host forty world leaders at the Nuclear Security Summit, on reducing the threat of nuclear terrorism.
But one leader who won't be there is Australia's Prime Minister Kevin Rudd, fuelling concern he's undermining his own nuclear disarmament commitments.
Presenter: Linda Mottram
Speakers: Barack Obama, U-S President; Kevin Rudd, Australia's Prime Minister; Rory Medcalf, director, International Security Program, Lowy Institute for International Policy
MOTTRAM: President Obama returned to Prague to sign the new Strategic Arms Reduction Treaty with President Medvedev of Russia. Some say it will deliver the largest verifiable nuclear weapons cuts in twenty years. Others caution it's just a different way of counting them. Either way it's important at least as an appearance of progress after President Obama stood before the same city a year ago and declared his global nuclear disarmament ambitions.
OBAMA: So today, I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons.
MOTTRAM: The issue has been close to the heart too of Australia's Prime Minister Kevin Rudd. In mid-2008, after laying a wreath at the Hiroshima war memorial in Japan he too spoke of a world free of nuclear weapons. He also announced a joint Australia-Japan initiative creating the International Commission on Nuclear Non-proliferation and Disarmament. Since then, he has shared discussions with Barack Obama on a nuclear disarmament path. But Kevin Rudd won't be at President Obama's Nuclear Security Summit on the 12th and 13th of April.
RUDD: It's important and I congratulate President Obama for his leadership in convening that summit, but my job here is to deliver health and hospital reform for all Australians. This is a once in a generation opportunity.
MOTTRAM: There's an uncanny resonance here. President Obama recently delayed a planned visit to Australia because he was fighting to get key domestic health reform legislation through Congress. Now, Kevin Rudd is battling Australian state leaders to get support for his health reforms, and he has a lot of ground to make up before a key meeting with them all on the 19th of April.
Still, some say it's crucial that Mr Rudd goes to Washinton next week. Rory Medcalf is director of International Security at the Lowy Institute for International Policy.
MEDCALF: Obama is going to need all the help he can get in tightening controls on nuclear exports to prevent the risk of nuclear materials falling into terrorist hands and to tighten the net around Iran as it seeks to acquire nuclear weapons. The second reason is Australia's wider credibility in the nuclear debate. The fact that Kevin Rudd himself commissioned a landmark study on nuclear disarmament and yet it seems is not turning up when the recommendations from that study are on the table. And the third reason is Australia's reputation as a uranium supplier. Australia has the world's largest deposits of uranium. It would appear to be irresponsible for Australia not to be represented at leadership level when the full security implications of the nuclear fuel cycle are being debated.
MOTTRAM: In terms of the summit it's been called because President Obama says that nuclear terrorism is the most immediate and extreme threat to global security. What in practical terms is likely to come out of it to address that threat?
MEDCALF: The summit won't be the end of the solution at all. But the aim is to build a new consensus from very diverse countries on the need to tighten controls over the export of nuclear materials, the need to increase surveillance and intelligence co-operation against the risks that materials will be diverted to potentially terrorist organisations. And I think in the margins of the summit if not at the centre of the summit will be the question of Iran and North Korea, how to deal with these proliferating states.
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