The confrontation on the Korean peninsula gives every appearance of reaching critical mass. The war clouds wafting from North Korea carry the danger of a violent episode if not of a second Korean War.

The question now is what the United States and others can do besides talk of “consequences,” as Secretary of State Hillary Rodham Clinton has warned, and of “saber-rattling,” as the White House said dismissively.

The standoff lends itself to choices that are potentially dangerous, unreliable – and possibly disastrous. One of these is the much-discussed Proliferation Security Initiative, or PSI, which empowers participants to try to blockade or at least monitor shipments of missiles and biological, chemical or nuclear materiel to and from other rogue states as well as terrorist organizations. The group has 16 “core” countries, while the remaining 80-plus nations have basically observer status.

In the wake of North Korea’s underground nuclear test last week, South Korea joined PSI as a core member. North Korea responded at once with a warning of military reprisals if anyone attempted to stop or board one of its vessels.

Despite Pyongyang’s rhetoric, the initiative may be of only symbolic value in terms of the defense of the Korean peninsula. Although PSI could form the basis of a de facto alliance, the participants so far have engaged mainly in exercises and exchanges of information.

This is not to say that PSI serves no real purpose. It is quite possible that nations, acting in concert, could use it to keep weapons of mass destruction, and the means to make and fire them, out of the hands of al-Qaida and Taliban terrorists.

For now, however, the Obama administration, while wielding the abstraction of PSI as a tool for countering proliferation, will have to settle on diplomatic pressure.

The first task is to strengthen sanctions against North Korea, getting China and Russia to stop the sale of all arms and spare parts to the North Korean army. China, as the major source of food, fuel and fertilizer for North Korea, could induce the North to knock off the launches and nuclear tests and focus on the overwhelming question of feeding its hungry people and restoring its dilapidated economy.

China is extremely reluctant to act firmly, but its hugely favorable trade balances with the United States and South Korea may be considerations that Beijing should not ignore.

China and the rest of the world have another, more disturbing, reason for cooperating to get North Korea to give up its nuclear program. That’s the danger of a nuclear arms race in the region.

Optimistically, it’s possible to imagine China agreeing that cooperation against proliferation is a fine idea. Pessimistically, China might view Japan and Taiwan as enemies against which it would need to build its defenses in league with North Korea. In such a scenario, South Korea would be exposed, and a regional war is not hard to imagine.

In the end, threatening North Korea with undefined “consequences” or with PSI actions isn’t going to persuade Pyongyang to calm down and return to talks on abandoning its nuclear weapons program. The Obama administration, like its predecessor, will have to rely on diplomatic pressure.