In a memo Saturday to NBC News staff that was released by the network, the anchorman said that as managing editor of "NBC Nightly News" he is taking himself off the broadcast for several days. Weekend anchor Lester Holt will fill in, Williams said.
NBC News refused to comment Saturday on when or whether Williams would return and who would decide his future.
Williams, however, said he would be back.
"In the midst of a career spent covering and consuming news, it has become painfully apparent to me that I am presently too much a part of the news, due to my actions," Williams said in his memo.
"Upon my return, I will continue my career-long effort to be worthy of the trust of those who place their trust in us," he wrote.
NBC News President Deborah Turness said Friday that an internal investigation had been launched after questions arose over Williams' false on-air statements that he was in a helicopter hit by a rocket-propelled grenade while in Iraq in 2003. Williams apologized for those statements Wednesday.
There was no indication by Williams, who has anchored "NBC Nightly News" since 2004, that an absence was forthcoming during his newscast Friday. He signed off as he usually does, saying he hoped people would be back to see him Monday.
Holt did mention Williams' leave in Saturday's newscast.
"A word tonight about our colleague Brian Williams, who you may know has been under scrutiny this past week over his recollection of certain stories he's covered," Holt said before reading Williams' memo to viewers.
Since Williams' apology, questions also have been raised about his claim that he saw a body or bodies in the Hurricane Katrina floodwaters that hit New Orleans in 2005.
His remarks in a 2006 interview drew suspicion because there was relatively little flooding in New Orleans' French Quarter, the area where Williams was staying. A person at NBC confirmed that Williams stayed at the Ritz-Carlton, which is in an area where a news photographer and a law enforcement official said they saw bodies.
The effect on "Nightly News" remains to be seen. But even if its ratings suffer for a long period, it wouldn't be enough to damage the credit profile of parent Comcast Corp., said Mike Simonton, an analyst with Fitch Ratings.
"Comcast is a large, diverse media conglomerate with meaningful financial cushion to endure weakness in any one of its smaller divisions over a period of time," he said. Other media companies have survived departures of on-air stars, Simonton said.
Paul Levinson, professor of communications and media studies at Fordham University, called Williams' time off a good idea for him and NBC News.
"It gives him a chance to catch his breath and, on a human level, it must be excruciating to get on the air and report the news and not say anything about this," Levinson said.
NBC News, he said, "wants to be in the business of reporting on the news, and not have people thinking, 'is he telling the truth?'"
Rich Hanley, director of the graduate journalism program at Quinnipiac University, also lauded Williams' leave.
It buys time for the network to assemble a contingency plan in the event it determines to suspend or remove him, Hanley said. A final decision may await the finding of any "collateral impact" on another key NBC program, "Today," which would be evident when the February ratings numbers come out, he said.
"Nightly News" has reigned as the top-rated evening newscast over its competition on CBS and ABC.
Williams' importance to NBC News goes beyond his anchor status, said Al Tompkins, a faculty member for broadcast and online at The Poynter Institute, a journalism think-tank .
"He sets the tone of the network. It may not be as critical as when (Walter) Cronkite was CBS' anchor, in every way, but he is more than a face," he said.
Williams' absence itself is a delicate challenge, according to Tompkins.
"He can't be gone long. The timing will be critical — too short and it won't seem like he has taken himself out of the game long enough, and too long and he looks like damaged goods," he said.
AP Television Writer David Bauder and AP Business Writer David Koenig in New York contributed to this report.