THE HAGUE, Netherlands -- Japan will hand over to the United States more than 700 pounds (315 kilograms) of weapons-grade plutonium and a supply of highly enriched uranium -- a victory for U.S. President Barack Obama's efforts to secure nuclear materials around the world.

American and Japanese officials announced the deal Monday at a two-day nuclear security summit in The Hague -- the meeting's first major breakthrough.

"This is a very significant nuclear security pledge and activity," U.S. Energy Secretary Ernest Moniz told reporters in The Hague. "The material will be transferred to the United States for transformation into proliferation-resistant forms."

Japan originally received the material from the U.S. and Britain in the 1960s for use in research.

The summit, part of a process launched by Obama in 2009, focuses not on nuclear weapons but on efforts to reduce and secure nuclear material stockpiles to prevent them falling into the hands of terrorists who could potentially use them to fashion a weapon or "dirty bomb."

The number of countries possessing such stockpiles has fallen from 39 in 2009 to 25 at the start of the Hague summit.

The summit which hosts leaders and senior officials from 53 countries, is scheduled to wrap up Tuesday with a closing communique in which they will pledge to further reduce and secure their nuclear stockpiles before a final summit in Washington in 2016.

Obama flew into the Netherlands Monday morning and was attending a hastily arranged G-7 summit later in the day to discuss the West's response to Russia's annexation of Crimea.

The White House also said that in addition to the Japan deal, the United States has reached agreements with Belgium and Italy to remove highly enriched uranium and plutonium from those European allies.

The White House said it had removed a "significant amount" of nuclear material from Belgium and about 20 kilograms from Italy. It did not give further timings or specifics of the operations.

"Italy and the United States plan to continue to work together to eliminate additional stocks of special nuclear material to make sure they do not fall into the hands of terrorists," the Obama administration said in a statement.

Kenneth Brill, a former U.S. ambassador to the International Atomic Energy Agency, welcomed the Japan deal.

"Any time you can start to strengthen security around any fissile material it is a good thing," he said.

Yosuke Isozaki, a senior national security adviser to Japanese Prime Minister Shinzo Abe, said handing over the highly enriched uranium and plutonium is part of Japan's efforts to prevent proliferation and possible abuse of nuclear material by terrorists -- the main aim of the Hague summit.

"Japan shares a vision of a world without nuclear weapons," he said through a translator.

As part of the deal, the U.S. will continue to receive spent reactor fuel from Japanese nuclear plants for an additional 10 years.

Isozaki said Japan and U.S. also "have agreed to conduct cutting-edge research together with alternative fuels."

Miles Pomper, a nuclear expert at the Monterey Institute of International Studies, called the return of the materials "a step in the right direction."

In addition to the weapons-grade material Japan is giving back, it maintains a stockpile of 9.3 tons of lesser-grade plutonium that could be easily weaponized by a country of Japan's sophistication, perhaps in a matter of months. That material also could present an attractive target for terrorists.

Additionally Japan's new Rokkasho nuclear plant, due to come online this year, is capable of producing almost that many more tons of plutonium per year when operational. Yet, with most of Japan's nuclear plants still shut down in the wake of the 2011 Fukushima nuclear plant disaster, there is no apparent use for that material.

"So this is a step forward, but it's not enough," Pomper said.