Washington, D.C. ? Robert Joseph, the State Department’s senior arms control and security official, has resigned.

Joseph submitted his resignation to President Bush amid uncertainty about the future of negotiations to curb nuclear programs in Iran and North Korea. Talks on both fronts have moved slowly, with U.N. penalties against both countries showing limited results. The talks are in recess.

Joseph, who will leave next month, acknowledged trying to stop Iran and North Korea from building nuclear weapons was very hard and he said they posed serious challenges to U.S. security.