A 47-nation summit in Washington agreed Tuesday to lock up the world's most vulnerable nuclear materials within four years to prevent terrorists from setting off a global "catastrophe."
The unprecedented gathering met the challenge posed by President Barack Obama, who said the world was littered with poorly guarded fissile material that militant groups could use to build a horrific weapon.
"We welcome and join President Obama's call to secure all vulnerable nuclear material in four years, as we work together to enhance nuclear security," the leaders said in a joint communique due to be released shortly and seen by AFP.
They outlined voluntary, yet only partly defined measures to combat nuclear trafficking, including sharing information and detection, forensics and law enforcement expertise.
The leaders said they "recognize the need for cooperation among states to effectively prevent and respond to incidents of illicit nuclear trafficking."
They also underlined that the "essential role" in combating nuclear proliferation rests with the UN watchdog, the International Atomic Energy Agency.
Increased security must "not infringe upon the rights of states to develop and utilize nuclear energy for peaceful purposes and technology," summit participants said.
Hosting the largest summit in the United States in over six decades, Obama pressed China to back UN sanctions on Iran over its controversial nuclear program.
Amid mixed signals from Beijing, Chinese President Hu Jintao told the summit that Beijing "firmly" opposes atomic weapons proliferation, while backing civilian uses.
On what are commonly referred to as loose nukes, Obama pressed his guests "not simply to talk, but to act."
"Nuclear materials that could be sold or stolen and fashioned into a nuclear weapon exist in dozens of nations," Obama said.
He said radioactive material as small as an apple was enough to kill thousands of people.
"Terrorist networks such as Al-Qaeda have tried to acquire the material for a nuclear weapon, and if they ever succeed, they would surely use it," the president said.
"It would be a catastrophe for the world -- causing extraordinary loss of life, and striking a major blow at global peace and stability."
Mexico gave Obama's initiative a boost by agreeing to give up weapons-grade uranium. Ex-Soviet Ukraine and Canada made similar pledges on Monday after Chile's earlier moves.
Russian Foreign Minister Sergei Lavrov announced plans to spend up to 2.5 billion dollars to dispose of plutonium from its massive defense program.
Russia and the United States also signed a new protocol pledging to complete the disposal of 34 tonnes of excess weapons-grade plutonium each, enough to make 17,000 weapons.
In parallel with the drive against loose nukes, Obama used the summit to drum up support on the US drive to slap a fourth round of UN sanctions against Iran over its nuclear drive.
Washington accuses Tehran of seeking to produce nuclear weapons, but Iran insists it is only pursuing civilian nuclear power.
Obama discussed the sanctions with Hu, a crucial partner since China, a big economic partner of Iran, has veto power in the UN Security Council.
China's foreign ministry meanwhile reaffirmed its long-held skepticism about the need for sanctions, saying that "pressure and sanctions cannot fundamentally solve" the standoff.
But the White House was optimistic about chances of getting China on board.
German Chancellor Angela Merkel, who backs sanctions, said she was "very hopeful."
Obama also met with the leaders of Brazil and Turkey, both non-permanent members of the UN Security Council reluctant to support sanctions.
In his address to the summit, Hu gave few clues, saying China opposed nuclear weapons proliferation, while also underlining that China backs "the equal right of all countries to the peaceful use of nuclear energy."
The New York Times reported that Obama was offering to help China maintain steady fuel deliveries if sanctions against Iran led to a disruption of oil supplies.
Iran defiantly said it was organizing its own nuclear conference to be held in Tehran on Saturday and Sunday with foreign ministers from 15 countries.
A manual on securing stocks of separated plutonium and weapons grade uranium, as well as advice on how to dispose of the dangerous materials, was issued at the end of the Washington summit.
However, all the steps are voluntary and the plan for accomplishing the four-year plan remains sketchy.