Robert Joseph, the State Department's senior arms control and security official, has submitted his resignation to President Bush.

Joseph resigned 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. sanctions against both countries showing limited results.

His departure follows that of several other top diplomatic officials, including Philip Zelikow, who was a close adviser to Secretary of State Condoleezza Rice, Deputy Secretary Robert Zoellick, and John Bolton, U.S. ambassador to the United Nations;

Joseph joined the State Department in 2005 after working on Bush's national security council staff.

He has taken a strong view on needs to eliminate North Korean and Iranian nuclear programs and warned last month against terrorist groups and other nations acting against U.S. and commercial satellites.

"After six years of service I have decided it is time to resign," Joseph said in the letter obtained Thursday.