The United States says the focus of the nuclear security summit that will open in Washington later this month will be on seeking ways to prevent the transfer of nuclear materials.

State Department spokesman Philip Crowley told reporters Thursday that the summit will focus on devising measures to keep nuclear materials and nuclear knowledge secure and how to prevent such materials from getting into the hands of both state and nonstate actors.

He said that although the summit has a broader agenda than North Korea and Iran, the U.S. is seeking to strengthen the nonproliferation regime expressly because of the irresponsible behavior of the two countries.

Crowley also said that the U.S. has not considered inviting North Korean leader Kim Jong-il to the international nuclear summit.

Crowley said that only the leaders who have shown strong determination toward nonproliferation have been invited to the summit, noting that the North is a serial proliferator that has yet to show the seriousness of purpose that it is committed to the world's nonproliferation agenda.