Yukiya Amano, director general of the International Atomic Energy Agency, told a conference in Washington that unilateral action on nuclear security wasn't enough to deter global threats.
Iraq in October became the latest country sign onto the Additional Protocol of the Nuclear Non-Proliferation Treaty, making it the 119th country do back the measure.
Amano said there was "unfinished business" on nuclear security, however, because of the lack of support for the Amendment to the Convention on the Physical Protection of Nuclear Materials.
"The amendment's entry into force would make an important difference to global nuclear security by enhancing national security frameworks and international cooperation," he said.
The amendment hasn't entered into force, though it was agreed upon nearly a decade ago. The measure expands on the physical protection of nuclear material during international transport.
Through collective international efforts, said Amano, "we make it more difficult for criminals and terrorists to traffic nuclear and radioactive material across borders."