Calling for a “new mindset” among world leaders, President Obama on Tuesday opened the working session of his international nuclear summit with dire warnings about terrorist efforts to get their hands on nuclear weapons.

Networks such as Al Qaeda have tried to acquire the material for a nuclear weapon, the president told the assembly, predicting that if they were to succeed, “they would surely use it.

“Were they to do so, it would be a catastrophe for the world, causing extraordinary loss of life and striking a major blow at global peace and stability,” Obama said. “It is increasingly clear that the danger of nuclear terrorism is one of the greatest threats to global security, to our collective security.”

It was a dark reminder the president’s team has been sounding all week in public remarks, as well as in one-on-one conversations between Obama and other leaders during the last three days.

Obama opened with it Tuesday as the plenary session with leaders of 46 other nations and international agencies came to order for the president’s summit in Washington. At the top of the agenda is how to lock down nuclear material and arms to make sure terrorists can’t threaten the world community or individual nations with an atomic attack.

The White House is hoping that Obama will be able to announce specific steps toward the goal of nuclear security by the time of an afternoon news conference expected after the summit concludes.

Already, the leaders of Chile, Ukraine and Canada have promised to reduce their stockpiles of enriched uranium that can be used in the manufacture of nuclear weapons.

Summit organizers at the White House also hope to release a communique after the end of the meeting. One of the working provisions of the agreement would set a goal of securing loose materials around the world within the next four years.

Unclear at the start of business Tuesday morning is whether the leaders will move closer to supporting imposition of sanctions against Iran, an effort which Obama and Western allies are leading in hopes of curbing that nation’s nuclear program.

Two decades after the end of the Cold War, Obama said, the “cruel irony of history” is that the risk of nuclear attack has gone up even as the risk of nuclear confrontation between nations has decreased.

“Nuclear materials that could be sold or stolen and fashioned into a nuclear weapon exist in dozens of nations,” Obama said. “Just the smallest amount of plutonium -- about the size of an apple -- could kill and injure hundreds of thousands of innocent people.”

World leaders must do more than talk, he said -- they must act to promote security.

“It requires a new mindset,” Obama said, “that we summon the will, as nations, as partners, to do what this moment in history demands.”

cparsons@tribune.com