UN Secretary-General Ban Ki- moon Tuesday called for "urgent global action" to ensure nuclear materials safety and prevent nuclear terrorism.

In his address to the plenary session of the two-day Nuclear Security Summit held?in Washington?in the U.S. capital, Ban said securing nuclear materials and preventing nuclear terrorism are global challenges.

"Even one attack could inflict mass casualties and change our world forever. Such a prospect should compel all of us to strengthen our common defenses," Ban told the 47 world leaders gathered at the summit.

He said he was pleased that the communique of the summit highlighted the need to strengthen global norms and to achieve universal membership in key multilateral treaties aimed at preventing terrorist groups and non-state actors from gaining access to the most lethal weapons and materials known to man.

As Tuesday marks five years since the adoption of the landmark International Convention for the Suppression of Acts of Nuclear Terrorism by the United Nations General Assembly, Ban said he will convene a conference at an appropriate time in consultation with the parties to review its implementation and to facilitate further ratifications.

"Only 65 countries have ratified it. This is far from satisfactory," the secretary-general said.

On securing the fissile materials, Ban said there is an urgent need for accurate accounting and transparency of all stockpiles of fissile materials, including historical production.

"It is also imperative to have a reliable international instrument to keep production of fissile materials in check," Ban said. "Without a verifiable and legally binding fissile material treaty, other efforts will amount to only half-measures."

He said he has repeatedly urged the Conference on Disarmament ( CD) to immediately start negotiations on a treaty banning the production of fissile material for nuclear weapons or other nuclear explosive devices, proposing to convene a meeting of CD at the ministerial level during this year's General Assembly session in September to regenerate much-needed political will.

Ban also suggested strengthening the role of the International Atomic Energy Agency and increasing the engagement of the Security Council.

"While there is much the world can do to reduce the risks posed by nuclear weapons, disarmament offers the greatest possible guarantee," Ban said.

He urged all states that have not yet ratified the Comprehensive Nuclear-Test-Ban Treaty to do so promptly and called on all parties to next month's five-yearly review conference of the 40-year-old Nuclear Non-Proliferation Treaty (NPT) to strengthen it.

"Attendance at the highest possible level would send a strong message to the world's people that we are fully committed to the treaty and serious in moving towards a nuclear-weapon-free world," he said. "We cannot fail."