U.S. President Barack Obama is hosting more than 40 leaders in Washington for a conference on preventing terrorists from gaining nuclear bombs. (Richard Clement/Reuters)

World leaders and delegations from 47 countries gathered in Washington for U.S. President Barack Obama's two-day summit to reduce the threat of having a nuclear weapon fall into the hands of terrorists.

"It's impressive," Obama told reporters Monday.

"I think it's an indication of how deeply concerned everybody should be with the possibilities of nuclear traffic, and I think at the end of this we're going to see some very specific, concrete actions that each nation is taking that will make the world a little bit safer."

The goal of the meeting is to agree on an action plan for participants to secure all of their material that can be used in nuclear weapons within four years so that it is no longer vulnerable to theft.

Canadian Prime Minister Stephen Harper arrived Monday for the nuclear security summit in Washington. (Jonathan Ernst/Reuters)

It had its first tangible outcome when Ukraine announced it would give up its stockpile of weapons-grade uranium by 2012, most of it this year.

Canada announced Monday that highly enriched uranium currently held in Ontario will be sent to the U.S. to be converted into a form that can't be used for nuclear weapons.

Earlier on Monday, UN Secretary General Ban Ki-moon said that he would urge the leaders at the summit to resume negotiations on such a ban without delay. The UN-backed Conference on Disarmament in Geneva has long been considering such a ban.

But Pakistan has blocked the start of negotiations, arguing that it would put it at a permanent disadvantage to India with which it has fought three wars since independence in 1947.

Obama held a series of one-on-one meetings with leaders from China, Jordan, Ukraine, Armenia and Malaysia before the National Security Summit got underway.

Just days ago in Prague, where he signed a new arms control agreement with Russian President Dmitri Medvedev, Obama pondered what groups like al-Qaeda might do if they got their hands on the world's most dangerous weapons.

"Even if they don't use them, they would then be in a position to terrorize the world community," said Obama.

On Monday, John Brennan, Obama's counterterrorism chief, warned that al-Qaeda is vigorously pursuing material and expertise for a bomb.

He said the group is also is a threat to penetrate the nuclear establishments of countries like Pakistan, where al-Qaeda leadership resides, Brennan said.

"Al-Qaeda is looking for those vulnerabilities and facilities and stockpiles in different countries that would allow them to obtain the byproducts of nuclear reactors and materials that they can use, but also to go after those individuals that might have access to the materials, as well as individuals who have the expertise that they need to actually fabricate and improvise a nuclear device," he said.

But Pakistani physicist Pervez Hoodbhoy said he did not see the problem as a threat.

"Unfortunately, I do not see this concern either in Pakistan or India about nuclear terrorism," he said. "Both countries do not see the seriousness of this situation."

Germany sees "dirty bombs" as a more immediate threat than regular nuclear weapons. Speaking in Berlin ahead of her departure for the summit, Chancellor Angela Merkel said such bombs — using conventional explosives to spread radioactivity over a large area — "must not under any circumstances" fall into the hands of terror groups.

"We believe that the [International Atomic Energy Agency] must be strengthened; we are ready to pledge additional finances to make this happen," Merkel said.

Former Canadian MP and senator Douglas Roche, along with other members of the Canadian disarmament movement, met with Harper on Friday. They sought a promise from Harper to support a broader and more binding treaty on nuclear disarmament.

'Most opportune moment'

"This is certainly the most opportune moment that I have experienced in my lifetime toward the concrete elimination of nuclear weapons," said Roche.

"We are not a bunch of naive people who think this can be done overnight, but the failure to start down the road toward verifiable phased reduction of nuclear weapons with a visible intent to get to zero will mean we are going to have the status quo prevail," he said.

While Canada has spent over $300 million to help retire Soviet-era nuclear weapons, one nuclear proliferation expert said Obama's summit is the perfect place to start shifting the focus toward new threats.

Trevor Findlay of Ottawa's Carleton University said those new threats include countries that might have dual-use material — nuclear material for peaceful purposes that could be used in weaponry — or might be prone to nuclear smuggling.

With files from The Associated Press