A two-day global conference on nuclear security, which opens on Monday in Washington, will focus on the vulnerability of stockpiles of nuclear materials and securing them against the threat of terrorist acquisition.

During the 47-nation summit, world leaders are expected to make concrete commitments to ensure that their national stocks of separated plutonium and enriched uranium are not stolen, smuggled or sold to terrorists.

The new U.S. nuclear doctrine has identified the procurenent of nuclear materials by extremist organizations as a major threat to global nuclear security.

"The threat of nuclear war... has diminished. The threat of nuclear terrorism has increased," U.S. Secretary of State Hillary Clinton said on the eve of the summit.

The U.S. summit comes after the signing last week of a new strategic arms reduction treaty by U.S. President Barack Obama and Russian President Dmitry Medvedev in Prague.

The two countries, which possess about 90% of global arsenals of nuclear weapons, have agreed to reduce the number of nuclear warheads to 1,550 on each side and the number of deployed and non-deployed delivery vehicles to 800 on each side.

Russian presidential aide Sergei Prikhodko said on Sunday that "global nuclear security can only be ensured if all states have effective national [nuclear security] systems."

He also said all nuclear materials and installations in Russia were reliably protected.