Japan pledged Monday to return hundreds of kilos of weapons-grade uranium and plutonium given to Tokyo for research during the Cold War era, in the first major deal announced at a summit on nuclear security.

"By removing this nuclear material, we can prevent the risk of nuclear terrorism," Japan's special nuclear advisor, Yosuke Isozaki, said on the sidelines of the 2014 Nuclear Security Summit (NSS) in The Hague.

Washington has been pressing Tokyo over the return of weapons-grade material, stored at Japan's Fast Critical Assembly (FCA) northeast of Tokyo, which experts said made a tempting target for extremists.

According to reports, Japan has more than 300 kilos (660 pounds) of plutonium and almost 200 kilos of highly-enriched uranium 235, supplied by Britain and the United States and now stored at the FCA facility in Tokai about 140 kilometres (87 miles) from the capital.

Japan does not have the bomb but the material could be used to build dozens of nuclear weapons.

Monday's announcement is seen as a major victory in President Barack Obama's push to secure the world's radioactive civilian stockpiles.

"This is a very significant nuclear security pledge engaging hundreds of kilogrammes in weapons-usable material," US Energy Secretary Ernest Moniz told journalists.

"The material will be transported to the United States for conversion into proliferation-resistant forms," he said.

"It affirms that most research and development can be done without using weapons-grade material," Moniz added.

"This is a very significant announcement because it removes this nuclear material from a potentially dangerous situation," Miles Pomper, a senior researcher at the Monterey Institute of International Studies, told AFP.

"Although the FCA is a small mock-up facility to do nuclear tests, they have a lot of material stored there," he added.

Japanese media reports said Japan has strongly resisted returning the plutonium, saying it was needed for researching fast reactors.

Since last year, Japan and the United States have been actively discussing the matter, The Japan Times reported.

The third NSS being held in The Hague is Obama's brainchild after announcing in 2009 that "nuclear terrorism was one of the greatest threats to international security."

But the summit, set up to discuss ridding the world from nuclear terror, has been overshadowed by the escalating crisis in Ukraine, after Russia's absorption of the Crimean peninsula.

The NSS is to be officially opened by Obama later on Monday.