Ukraine has agreed to eliminate its stockpile of weapons-grade nuclear material, the US says, as a nuclear security summit opens in Washington.
The White House said Ukraine would by 2012 get rid of enough highly enriched uranium to build "several weapons". Leaders from 47 countries are taking part in the summit, called by President Barack Obama.
Before the talks, Mr Obama and China's President Hu Jintao agreed to step up pressure on Iran over its atomic plans.
 |  This is an unprecedented gathering - Mr Obama will hope for an unprecedented outcome  Jonathan Marcus BBC diplomatic correspondent  |
Mr Obama said earlier that a clear message needed to be sent to Tehran over its nuclear ambitions.
Ukraine's move to get rid of all of its stockpile of highly enriched uranium by the time of the next nuclear security summit in 2012 was hailed as a "landmark decision" by the US.
"This is something that the United States has tried to make happen for more than 10 years," said White House spokesman Robert Gibbs. "The material is enough to construct several nuclear weapons."
Al-Qaeda threat?
This summit is all about securing stocks of fissile material - highly enriched uranium and plutonium - that could potentially be used by terrorists to build a nuclear bomb, says the BBC's diplomatic correspondent, Jonathan Marcus, in Washington.
He says the announcement of Ukraine's decision may have been choreographed, but it is just the sort of news Mr Obama wants to hear as he launches the summit.
World leaders have been arriving in Washington ahead of the summit
Ukraine's agreement sets a precedent that Mr Obama would like other countries to follow, our correspondent adds.
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.
There are also about 500 tonnes of the other key ingredient of a nuclear weapon - plutonium.
In total, that is enough to make 120,000 nuclear weapons.
Much international, largely US-funded, effort has attempted to reduce the threat of nuclear leakage from Russia in particular, but it remains a concern.
Mr Obama has also highlighted the danger of groups like al-Qaeda getting hold of nuclear devices.
New nuclear policy
Before opening the summit, President Obama held meetings with a handful of international leaders.
The White House said in a statement that he had also discussed Iran with Malaysian Prime Minister Najib Razak.
|  Perhaps some threats seem too much like fiction to be taken seriously Mark Mardell BBC's North America editor |
Among those attending the summit are Israel, India and Pakistan - three nations that have not signed the non-proliferation treaty.
Neither North Korea nor Iran, two states with disputed nuclear ambitions, have been invited. The two countries are viewed by the US as violators of the non-proliferation agreement.
Syria was also left off the invitation list because the US believes Damascus has nuclear ambitions, according to the Associated Press news agency.
Israel is being represented at the summit only by a deputy prime minister, amid reports that its government is worried that Turkey and Egypt might use the occasion to raise the issue of its suspected nuclear arsenal.
Last week, the US and Russia signed a new Strategic Arms Reduction Treaty, reducing each country's deployed nuclear arsenal to 1,550 weapons.
Earlier in the week, Mr Obama approved a new nuclear policy for the US, saying he planned to cut the nuclear arsenal, refrain from nuclear tests and not use nuclear weapons against countries that did not have them.
BBC
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