Writing in the New York Times on Thursday, Muhammad al-Baradai said that nuclear technology is widely accessible now through "a sophisticated worldwide network able to deliver systems for producing material usable in weapons".
Al-Baradai echoed US President George Bush's call in a speech on Wednesday for states to tighten up control of their companies' nuclear exports to proliferators.
Al-Baradai, the International Atomic Energy Agency (IAEA) director-general, said the world must act quickly because inaction would a create a proliferation disaster.
"The supply network will grow, making it easier to acquire nuclear weapon expertise and materials.
"Eventually, inevitably, terrorists will gain access to such materials and technology, if not actual weapons," he wrote.
"If the world does not change course, we risk self-destruction," al-Baradai said.
The father of Pakistan's atom bomb, Abd al-Qadir Khan, admitted last week that he and fellow scientists leaked nuclear secrets.
Khan has admitted selling secrets
to Iran, Libya and North Korea
They are believed to have been part of a global nuclear black market organised to help countries under embargo such as Iran, North Korea and Libya skirt international sanctions and obtain nuclear technology that could be used to make weapons.
The massive illicit network has touched on at least 15 countries around the world.
Al-Baradai said the 1968 nuclear Non-Proliferation Treaty (NPT), the global pact aimed at stopping the spread of atomic weapons, needed to be revisited and toughened to bring it in line with the demands of the 21st century.
He said it should not be possible to withdraw from the NPT, as North Korea did last year, while the tougher inspections in the NPT Additional Protocol should be mandatory in all countries.
Currently fewer than 40 of the more than 180 NPT signatories have approved the protocol.
Al-Baradai said that the Nuclear Suppliers Group (NSG), a 40-nation group of countries that work together to prevent the export of peaceful nuclear technology to countries that might want weapons, needed to be transformed into a binding treaty.
"The supply network will grow, making it easier to acquire nuclear weapon expertise and materials. Eventually, inevitably, terrorists will gain access to such materials and technology, if not actual weapons. If the world does not change course, we risk self-destruction"
"The current system relies on a gentlemen's agreement that is not only non-binding, but also limited in its membership: it does not include many countries with growing industrial capacity," he wrote.
"And even some members fail to control the exports of companies unaffiliated with government enterprise," he added.
Al-Baradai called for the production of fissile material for weapons to be halted and enrichment technology restricted.
He said people who assist proliferators should be treated as criminals and states should eradicate loopholes that enable sensitive exports to slip past regulators.
He also called on the atomic weapons states who signed the NPT - the United States, China, Russia, Britain and France - to move towards disarmament as called for in the pact.
In a clear jab at the US, which plans to forge ahead with research into the so-called mini nukes, Al-Baradai said the world must drop the idea that nuclear weapons are fine in the hands of some countries and bad in the hands of others.
"We must abandon the unworkable notion that it is morally reprehensible for some countries to pursue weapons of mass destruction yet morally acceptable for others to rely on them for security - and indeed to continue to refine their capacities and postulate plans for their use," he said.