The top U.S. official for North Korea policy resigned last week, multiple sources said Tuesday.Jung Pak left the post as the U.S. senior official for North Korea and a process is under way to select her replacement, according to the sources.She has led a series of efforts to strengthen cooperation between South Korea and the United States, and among South Korea, the U.S. and Japan over North Korea-related issues.Source: Yonhap News Agency