WASHINGTON – President 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: The White House counterterror chief warned that al-Qaida is vigorously pursuing ingredients and expertise for a bomb.

At the same time, Obama and Chinese President Hu Jintao sought agreement on potential sanctions to discourage Iran’s efforts to come up with its own nuclear weapon.

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 today, even if the means to accomplish it are unclear. The fear is that terrorists or a nation other than the major nuclear powers could obtain crucial ingredients and inflict horrendous damage.

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.

In a brief exchange with reporters at the White House, 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 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.

According to Harvard’s Belfer Center, there are about 3.5 million pounds of highly enriched uranium and half a million pounds of bomb-grade plutonium in the world. Combined, they could be used to build as many as 200,0000 nuclear weapons, or about 8½ times the world’s current total of 23,360 warheads.

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.

In a detailed report on the challenge of securing nuclear materials, Bunn said no one really knows the likelihood of nuclear terrorism.

“But the consequences of a terrorist nuclear blast would be so catastrophic that even a small chance is enough to justify urgent action to reduce the risk,” he wrote. “The heart of a major city could be reduced to a smoldering radioactive ruin, leaving tens to hundreds of thousands of people dead.”

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.”

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-Qaida 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.

Another major concern of many attendees at the conference is that Iran will build or obtain a nuclear weapon.

Iran was a major topic of a lengthy meeting between Obama and Hu, said White House national security aide Jeff Bader. “They’re prepared to work with us,” Bader said of the Chinese position on Iran, a stance he called a sign of international unity.

He said Obama and Hu agreed that a set of potential sanctions should make clear to Iran the cost of continued nuclear defiance. Yet there was no breakthrough, and Chinese spokesman Ma Zhaoxu did not mention sanctions in his more-general statement on the meeting.

“China and the United States share the same overall goal on the Iranian nuclear issue,” the Chinese statement said.

Iran’s U.N. Ambassador Mohammad Khazee on Monday declared Obama’s new nuclear policy, which excludes Iran from a U.S. pledge not to use nuclear weapons against countries that do not have them, an act of “state terrorism” because it threatens nations with weapons of mass destruction.

“This policy of nuclear blackmail and terror” runs counter to international law and the U.N. Charter and “must be denounced by the international community and all peace-loving nations,” Khazee told a General Assembly committee considering a draft Comprehensive Convention on Terrorism.

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, press secretary Robert Gibbs said.