Taiwan's recent seizure of 158 barrels of dual-use chemicals from a North Korean freighter highlights the international community's increased determination to counter the proliferation of weapons of mass destruction (WMD). Wisely, the Bush administration has shown increasing appreciation of the need to supplement its unilateral counter-proliferation measures, such as the invasion of Iraq, with cooperative efforts directed at other countries. For example, the administration recently secured an agreement among the Group of Eight (G-8) leading industrial states to step up efforts to curb WMD proliferation and combat the threat of "dirty bombs." (Dirty bombs are conventional weapons that disperse radioactive materials.)

Even more important has been the newly launched Proliferation Security Initiative (PSI) which President Bush announced on May 31, 2003. The Initiative seeks to promote international agreements that would enable the United States and other countries to interdict planes and ships suspected of carrying WMD, missiles, and their related equipment and technologies. Unlike existing national export controls, the PSI aims to impede WMD trafficking directly between countries of proliferation concern, like North Korea and Iran.

The PSI, thus far, has involved Australia, France, Germany, Italy, Japan, the Netherlands, Poland, Portugal, Spain, the United Kingdom, and the United States. Their first meeting occurred in Madrid on June 12, and their second gathering took place in Brisbane, Australia, on July 9-10. The attending governments stressed the need for effective information-sharing among national authorities, and agreed to conduct a series of interdiction training exercises involving air and naval forces as well as civilian agencies. The administration decided it could make more rapid progress launching the new policy if initially only the most important and active countries (in terms of nonproliferation) participated. President Bush has said the United States would eventually seek to "extend this partnership as broadly as possible."

The Bush PSI presently does not seek to change international law. Instead, it aims to determine how best to exploit the existing domestic legislation of the participating countries to interdict shipments of WMD, missiles, and their related equipment and technologies. U.S. officials argue that current national and international laws are adequate, provided they are enforced better and with greater international coordination. They anticipate that extensive consultations in advance of an event should make responding to actual interdiction incidents much easier. Accordingly, discussions have centered on seizures that could occur inside territorial waters (which extend 12 miles from countries' shorelines) and national airspaces.

But at some point the member countries may need to consider changes to international laws and practices, as proliferators will naturally seek out transit routes that circumvent the territories of PSI participants. Under existing international maritime law, it is generally illegal to stop and search foreign ships sailing in international waters, but national authorities can board vessels with the permission of the country under whose flag the ship is sailing, or if a ship lacks a flag. They can also seize vessels with illicit cargo-such as narcotics or contraband-or that are conveying goods to or from countries in violation of UN sanctions. Unfortunately, current international law does not ban shipments of WMD or their components. Countries concerned about such commerce have had to form separate suppliers' groups and other institutions that rely on national means of enforcement. Existing international agreements also permit countries not party to the NPT-such as India, Israel, Pakistan, and perhaps North Korea-to ship nuclear material. In addition, the NPT allows its signatories to import and export nuclear materials if they allow monitoring by the International Atomic Energy Agency (IAEA).

The administration should seek to expand the roster of PSI participants sooner rather than later. At a minimum, PSI partners will need to convince such important source countries as Russia and China to join with them if they wish to curb illegal weapons shipments. At least passive Russian and Chinese support would be required for the UN Security Council to pass a resolution supporting the Initiative. Such endorsement of the PSI as a whole, rather than of each individual enforcement action on a case-by-case basis, might help broaden enforcement efforts. Furthermore, the United States and its PSI partners should seek to enlist countries that are located along known proliferation routes and chokepoints, or that serve as major international transit hubs or facilitators. The latter category would include a country like Panama, since an estimated 11 percent of registered cargo ships in 2002 flew the Panamanian flag, according to Lloyd's Register, an independent organization that compiles shipping statistics used by the International Maritime Organization. Increasing South Korea's involvement in PSI also would be essential, given its proximity to North Korea.

Unfortunately, only limited success can be expected from attempts to improve interdiction of WMD and their associated components and delivery systems. Detecting chemical and biological weapons materials is difficult, and their components often have legitimate civilian uses. For example, to stop all potential WMD materials from entering Iraq after the 1991 Gulf War, the Security Council passed a resolution that barred the import of thousands of dual-use items. Furthermore, only relatively small amounts of fissile material are needed for a simple nuclear weapon, and these amounts can be concealed in radiation-proof containers. Analysts warn that Iraq, Iran, and North Korea have been able to exploit a new proliferation distribution network of commercial agents and less developed countries such as Pakistan to circumvent existing legal and technological safeguards.

By all means, the administration should proceed with the PSI, as well as with efforts to strengthen such counter-proliferation institutions as the Nuclear Suppliers Group, the Missile Technology Control Regime, the Biological Weapons Convention, and the Australia Group. Steps should also be taken to sustain the ability of the United Nations to dispatch a corps of trained inspectors to monitor states that have been prohibited from acquiring WMD or ballistic missiles, as well as to verify agreements having the same purpose. But interdiction, monitoring, and verification will need to be supplemented by other defensive and active measures-including global and regional diplomatic initiatives, effectively targeted sanctions, and, when necessary, preemptive military action-aimed at curbing WMD and missile proliferation and deterring states from using any WMD they do acquire.

Richard Weitz is a Senior Staff Member at the Institute of Foreign Policy Analysis.