President Barack Obama optimistically opened a 47-nation nuclear summit Monday, boosted by Ukraine's announcement that it will give up its weapons-grade uranium. More sobering: Obama's counterterrorism chief pointedly warned that al Qaeda is vigorously pursuing material and expertise for a bomb.
Ukraine's decision dovetailed with Obama's goal of securing all vulnerable nuclear materials worldwide within four years - an objective that the White House hopes will be endorsed by all summit countries at a closing session Tuesday, even if the means to accomplish it are unclear.
Before formally opening the summit with a reception and working dinner, Obama held a series of one-on-one meetings with leaders from China, Jordan, Ukraine, Armenia and Malaysia. Presidential aides billed the summit as the largest gathering of world leaders hosted by an American president since the 1945 conference in San Francisco that founded the United Nations.
In a brief exchange with reporters, Obama said of the summit: "It's impressive. 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 president has set a goal of ensuring all nuclear materials worldwide are secured from theft or diversion within four years. Analysts say meeting that ambitious goal is unlikely, reports CBS News correspondent Sharyl Attkisson, but this week's summit is viewed as a key milestone along the way.
CBS News senior White House correspondent Bill Plante reports there are an estimated 3.5 million pounds of highly enriched uranium scattered across 40 countries, and just 55 pounds of the substance is sufficient to make a small nuclear device.
While the president's goal of eradicating that threat within four years is lofty, Sharon Squassoni, a Senior Fellow at the Center for Strategic and International Studies tells CBS News it might not be completely unattainable.
"Locking down all of this material in four years is a tough goal... but if all of these states - and some of them really do have nuclear security problems - if they really commit to working on it, I think it's achievable," said Squassoni.
Security has been stepped up in Washington at hotels, embassies and meeting sites for the nuclear summit, reports Attkisson.
The talks are a centerpiece of Obama's broader agenda for ridding the world of nuclear weapons, a long-term process that he says should include gradual disarmament by the nuclear powers, stronger steps to head off a nuclear arms race and more urgent action to lock down tons of plutonium and highly enriched uranium that are the key building blocks of atomic weapons.
Not all countries share Obama's view that the nuclear-materials problem is a priority. Some think the bigger emphasis should be on disarmament, particularly by the United States and Russia, which despite recent reductions still possess 90 percent of the world's nuclear arms.
At a parallel unofficial conference of more than 200 international nuclear experts, participants said too many leaders don't share Obama's urgency about nuclear ingredients.
"There is a great complacency among policymakers around the world that terrorist groups couldn't make a nuclear bomb," said Matthew Bunn of Harvard University.
Secretary of State Hillary Clinton told CBS News chief Washington correspondent Bob Schieffer the administration's , "is putting everybody on notice… We don't want more countries to go down the path that North Korea and Iran are."
The nuclear posture review may have removed some of the intentional ambiguity from U.S. nuclear policy, but it does not leave the country any less safe, Clinton and Secretary of Defense Robert Gates
"Obama is banking on a new multilateralism to stem the flow of money and technology to Iran, North Korea and to terrorists," CBS News foreign affairs analyst Pamela Falk writes from United Nations Headquarters.
"The nuclear summit this week is a prelude to both the Senate ratification of the U.S.-Russia treaty, and the Nuclear Non-Proliferation Treaty conference next month at the U.N., at which Mr. Obama is hoping to gain further support for new sanctions, with teeth, against Iran," says Falk.
Pakistani physicist Pervez Hoodbhoy focused on his home region, where both Pakistan and India are building up their nuclear arsenals.
"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."
Ahead of the conference, German Chancellor Angela Merkel made it clear that she, too, sees dirty bombs in terrorist hands as an even larger threat than regular nuclear weapons.
Merkel said Monday that such weapons "must not under any circumstances" fall into the hands of terror groups such as al Qaeda.
"We believe that the IAEA must be strengthened, we are ready to pledge additional finances to make this happen," Merkel said of the nuclear watchdog International Atomic Energy Agency.
Merkel praised Obama's decision to hold the conference as "extremely important," and said it fits well with Obama's global disarmament efforts.
Meanwhile, Russian President Dmitri Medvedev said in an interview on ABC television's "Good Morning America" on Monday that Iran's nuclear program must be watched closely, but he said sanctions on the regime would have to be smart and effective because sanctions often do not work.
"They should not lead to humanitarian catastrophe, where the whole Iranian community would start to hate the whole world," the Russian president said.
He added that there was no global consensus for sanctions on Iran's petroleum industry.
Seeking to highlight the urgency of the threat posed by terrorists in pursuit of a nuclear bomb, John Brennan, the White House's counterterrorism chief, told reporters that al Qaeda is actively in search of the key ingredients for a bomb and the expertise to assemble it.
He said such an improvised device could be obtained through criminal gangs or by infiltrating nuclear labs in Pakistan or other nuclear nations.
Defense Secretary Robert Gates said Obama's conference offers a way to reinforce existing global controls on nuclear materials.
"It's an area that people talk about a lot, but frankly there hasn't been the concerted international attention in these two areas that there might have been," Gates said at the Pentagon. "I think that it creates some real opportunities."
Ukraine, which gave a major boost to arms control in 1994 when it agreed to surrender the nuclear weapons it inherited in the collapse of the Soviet Union, agreed to get rid of its weapons-grade fuel by 2012, according to the White House. Some details are yet to be worked out, including how and where the fuel will be disposed of, White House press secretary Robert Gibbs said.
Gibbs said the material could be sent to the U.S. or Russia. He declined to specify the amount, other than to say it was enough to make several nuclear weapons.
After a private meeting, Obama and Ukraine President Viktor Yanukovych issued a statement in which Obama praised the agreement as historic and pledged U.S. technical and financial assistance to support it. No dollar amount was mentioned.
More on Obama's nuclear strategy:
Obama Signs Nuclear Arms Pact with Russia
Read the Treaty
Nuclear Posture Review Report(PDF)