Republican presidential candidate Ben Carson is standing by his belief that Egypt's pyramids were built by the biblical figure Joseph to store grain.

But experts say it is accepted science they were tombs for pharaohs.

Mr Carson is a retired surgeon who has unexpectedly risen to the top of the Republican primary race in some polls.

Video posted online this week shows Mr Carson explaining his theory 17 years ago at a Michigan college affiliated with his Seventh-day Adventist Church.

In the video, Mr Carson says: "My own personal theory is that Joseph built the pyramids in order to store grain."

He was referring to the Old Testament story of Joseph predicting famine and advising the pharaoh to store surplus food.

Mr Carson said that is more likely than the accepted archaeological conclusion that the massive structures were built as tombs for pharaohs.

At a book signing on Thursday in Florida, Mr Carson stood by his statement.

Calling the grain theory his "personal belief", Mr Carson said, "I happen to believe a lot of things that you might not believe because I believe in the Bible."

Neither Mr Carson's church nor any other major Jewish or Christian sect shares his belief about the pyramids' origins.

Jodi Magness, a specialist in biblical archaeology at the University of North Carolina-Chapel Hill, said she knows of no scholar or archaeologist who questions that the pyramids were used as royal tombs.