Former US first lady Michelle Obama says she was so disappointed by the election of Donald Trump that she "stopped even trying to smile" at his presidential inauguration ceremony.

Key points: Ms Obama says she will "never forgive" Mr Trump over the "birther" conspiracy

Ms Obama says she will "never forgive" Mr Trump over the "birther" conspiracy US President accuses Ms Obama of using "controversy" to drive book sales

US President accuses Ms Obama of using "controversy" to drive book sales Ms Obama's new book Becoming comes out this week

Ms Obama, whose husband Barack served two terms as president from 2009 to 2017, makes the revelation in her new book, Becoming, which comes out on Tuesday in the US.

In it she reveals her disappointment at Mr Trump's victory over Democratic candidate Hillary Clinton in the 2016 presidential race.

"The vibrant diversity of the two previous administrations was gone," Ms Obama says in audio of the book released by the American ABC network.

"Someone from Barack's administration might have said that the optics there were bad, that what the public saw didn't reflect the President's reality or ideals, but in this case, maybe it did.

"Realising it, I made my own optic adjustment. I stopped even trying to smile."

Trump 'put my family's safety at risk': Obama

Michelle Obama says Donald Trump put her family in danger by promoting a conspiracy theory about her husband. ( Reuters: Rob Carr )

Ms Obama also said she would "never forgive" Mr Trump for casting doubt over her husband's citizenship.

Mr Trump had been an outspoken promoter of the "birther" conspiracy that Mr Obama was born outside the US and therefore ineligible to be president.

Ms Obama said Mr Trump's comments had put her family in danger.

"What if someone with an unstable mind loaded a gun and drove to Washington?" she says in audio of the book.

"What if that person went looking for our girls?

"Donald Trump, with his loud and reckless innuendos, was putting my family's safety at risk. And for this, I'd never forgive him."

Mr Trump responded to the criticism, accusing Ms Obama of trying to drive book sales with "controversy".

"She got paid a lot of money to write a book and they always insist that you come up with controversy," Mr Trump said.

"I'll give you a little controversy back, I'll never forgive [Mr Obama] for what he did to our US military. It was depleted … and I had to fix it."