President Obama said on Sunday that he was not worried that increasing and significant numbers of Americans believe he is Muslim.

“I can’t spend all my time with my birth certificate plastered on my forehead,” Mr. Obama said in an interview with Brian Williams on “NBC Nightly News.” “The facts are the facts. And so it’s not something that I spend all my time worrying about. And I don’t think the American people want me to spend all my time worrying about it.”

Mr. Obama, who is Christian, said that as a candidate, and now as president, he has repeatedly dealt with persistent rumors about his religion and with doubts about whether he was born in the United States. A recent poll showed that nearly a fifth of the respondents believed he was Muslim.

The poll, conducted by the nonpartisan Pew Research Center and its affiliated Pew Forum on Religion and Public Life, found that 18 percent of the people surveyed thought Mr. Obama was Muslim, according to a report by The Associated Press.