Sixteen months into his presidency, 13 percent of Americans say that President Barack Obama wasn't born in the United States.

A total of 13 percent of Americans said that Obama was born in Kenya, Indonesia or "someplace else outside the U.S.," according to a new 60 Minutes/Vanity Fair poll released Monday.

Another 11 percent of Americans said they were unsure of the country in which Obama was born.

Concerns over the location of Obama's birth have driven the so-called "birther" movement, a movement that asserts Obama may be constitutionally ineligible as president if he were not born a U.S. citizen.

Obama released his birth certificate some time ago, which shows that he was born in Honolulu, Hawaii, on August 4, 1961. Birthers complain that Obama should release his "long-form" birth certificate, and some Republican lawmakers, who deny that they're birthers, have introduced legislation to require birth certificates from presidential candidates.

39 percent of those surveyed accurately named Hawaii as the birthplace of Obama, and 20 percent were unsure in which state the president was born. One percent said Kansas, where Obama spent some time in his youth, while three percent said he was born somewhere else in the United States.

For those who said Obama was born outside the U.S., six percent said he was born in Kenya, two percent said in Indonesia and five percent said someplace else outside the United States.

The poll of 960 adults was conducted June 1-3.