The leaders of almost 50 countries have pledged to secure all vulnerable nuclear material within four years.

核安全峰会上，将近50个国家的领导人承诺，在未来四年之内严密保护核原料。

US President Barack Obama said the joint¹ action（联合行动） plan agreed at a summit in Washington would make a real contribution to a safer world.

The plan calls for every nation to safeguard nuclear stocks and keep material out of terrorists' hands.

Earlier, Russia and the US signed an agreement to dispose of（处理，解决） 68 tonnes of surplus（剩余的，过剩的） weapons-grade plutonium（钚） .

The combined stockpiles（库存） - 34 tonnes from each country - are said to be enough to make 17,000 nuclear warheads. US officials said it would be used as fuel in civilian² reactors³ to generate electricity.

The US will provide $400m of the funding for the disposal of Russia's plutonium, which Moscow estimates will cost up to $2.5bn (£1.63bn).

Several other countries - including Mexico, Chile and Ukraine - had earlier agreed to give up their stocks of highly-enriched uranium.

'Non-state actors'

At the end of the unprecedented⁴（空前的） 47-nation nuclear security summit, Mr Obama announced that the assembled world leaders had agreed a work plan to counter the danger of nuclear material falling into the hands of terrorists.

"Today, we are declaring that nuclear terrorism is one of the most challenging threats to international security," he said.

"We also agreed that the most effective way to prevent terrorists and criminals from acquiring nuclear materials is through strong nuclear security."

"[This] is a testament⁵（遗嘱，圣约） of what is possible when nations come together in a spirit of partnership⁶ to embrace our shared responsibility and confront a shared challenge," he added.

In a joint communique（联合声明） , the leaders agreed to non-binding measures to "secure all vulnerable nuclear material in four years" and to "prevent non-state actors from obtaining the information or technology required to use such material".

They said they would co-operate more deeply with the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), and share information on nuclear detection and ways to prevent nuclear trafficking（非法交易） .

But increased security should "not infringe⁷ upon（侵犯，侵害） the rights of states to develop and utilise（利用，使用） nuclear energy for peaceful purposes and technology", they added.

Progress is to be reviewed at a summit in South Korea in 2012.

BBC diplomatic correspondent Jonathan Marcus, who is at the summit, says the deal depends upon the will of participating governments to take action.

There can be no guarantees（保证） of progress, but it is clear the Obama administration intends to keep this issue at the top of the global agenda, he adds.