Williams has canceled a planned appearance with late-night talk show host David Letterman this week, an NBC News source said on Sunday, the latest fallout over misstatements by Williams over his experience reporting on the Iraq war. Williams, a star anchor who has garnered high ratings for the network, said on Saturday that he would voluntarily take himself off the evening newscast for several days while NBC looks into an issue that has damaged his credibility.NBC, a unit of Comcast Corp, on Friday said it was launching an internal probe of Williams over his statements that he was aboard a helicopter in 2003 that was brought down by enemy fire during the U.S. invasion of Iraq.Williams, who has told the tale repeatedly over the years, has escaped close scrutiny over the remarks until recently. He apologized this week, saying he "misremembered" the incident, an explanation that prompted military personnel and other journalists to call for his resignation. Williams has also been widely derided on social media. Elizabeth Spayd, editor and publisher of Columbia Journalism Review, told ABC's News's "This Week with George Stephanopoulos" on Sunday that NBC should have an outside expert carry out an investigation of Williams' statements.NBC News declined comment over the Letterman cancellation.Reuters contributed to this report.