Today's Washington summit between President Barack Obama and South Korean President Lee Myung-bak provides a golden opportunity for the two allies to send a clear and unequivocal signal that Washington and Seoul are in lock step when it comes to dealing with Pyongyang.

The first and most important message to reinforce is that neither the U.S. nor South Korea accepts North Korea as a nuclear weapons state and that normalization of relations will remain impossible until such time as Pyongyang verifiably gives up its nuclear weapons. Their unshakable mutual goal is the complete, verifiable, irreversible denuclearization of the Korean Peninsula.

They should also endorse and reinforce the United Nations Security Council message that Pyongyang's current actions are unacceptable and that it must return to full compliance with all appropriate Security Council resolutions and directives. In the meantime, both South Korea and the U.S. will honor and enforce

all binding resolutions and will take steps to augment current economic sanctions. A jointly implemented targeted financial sanctions package will demonstrate their combined seriousness to Pyongyang, as would their challenge to Beijing (among others) to follow suit.

These measures are not aimed solely at punishing North Korea - although punishment is and should be part of the consequences for past violations - but most importantly to ensure that whatever nuclear capability or materials currently exist in North Korea stay in North Korea and do not find their way to others who might be inclined to use them. The U.S. and South Korea - and the Security Council - should more clearly warn Pyongyang that if nuclear weapons or materials are transferred to another state or entity, there will be a commensurate and credible response, to include the possible use of force.

Washington should reinforce that the U.S. will “;do what it must to provide for our security and that of our allies”; and that neither the alliance itself nor the presence of U.S. troops in South Korea will ever be on the negotiating table with the North - the fate of the alliance (including troop dispositions and command arrangements) is for the two alone to decide.

The Obama administration should also carefully consider if additional force deployments may be necessary to reinforce its defense commitment to South Korea (and Japan), not to threaten Pyongyang but to remind it that any hostile act of aggression on its part would be met with a “;clear and appropriate response,”; even while providing assurance that “;we have no intention to invade North Korea or change its regime through force,”; provided, of course, that Pyongyang similarly refrains from hostile actions.

Refraining from a “;regime change by force”; policy does not equate to an endorsement of the current Kim Jong-il regime. The two presidents should make it clear that their support for, and willingness to work with, the current or any future North Korean regime is and will continue to be contingent on that regime's demonstrated willingness to give up its nuclear weapons in return for positive security assurances (in advance) and economic assistance (upon proven compliance with past and future agreements). They need to remind others (and themselves) that the Six-Party Talks are not the objective, but merely one possible means of achieving the long-term goal, which remains complete Korean Peninsula denuclearization. They should also repeat the pledge initially made by their more distant predecessors (Presidents Bill Clinton and Kim Young-sam) that the U.S. will enter into no negotiations dealing with the future security of the Peninsula that do not also involve South Korea - this does not preclude direct bilateral U.S.-North Korea negotiations on denuclearization or normalization issues but does preclude any return to the old 1994 Agreed Framework format where Washington and Pyongyang bilaterally reached an agreement that Seoul (and Tokyo, among others) then were expected to help finance.

The two presidents also need to help close the glaring loophole inherent in prior approaches to Pyongyang. In 2006, the Security Council imposed a series of sanctions against Pyongyang in reaction to its first nuclear test only to turn a blind eye toward enforcement once the North returned to the talks. Pyongyang no doubt assumes that whatever measures are put forth in response to its most recent (or planned future) missile or nuclear tests will likewise be ignored once it finally decides to come back to the negotiating table.

The two presidents understand, and should acknowledge, that future North Korean provocation is likely. As a result, they should announce the start of joint consultations - preferably with the Japanese as well - regarding next steps that they will, and that the Security Council should, take if and when future violations of Security Council resolutions occur. These should include mandatory enactment and enforcement of financial and other economic sanctions not just until Pyongyang rejoins six-way talks but until it comes into compliance with all existing resolutions.

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Ralph A. Cossa is president of the Pacific Forum CSIS (.(JavaScript must be enabled to view this email address)), a Honolulu-based nonprofit research institute affiliated with the Center for Strategic and International Studies in Washington. A longer version of this article originally appeared in the Korea Herald.