Bob Woodward's new book, "Fear: Trump in the White House," has dropped bombshells about the White House.

President Donald Trump has condemned the book over its use of anonymous sources, but Woodward told The New York Times there was "no alternative" if the public wanted the truth.

In an episode of the Times podcast "The Daily," Woodward said a key official had phoned him to say the book was "1,000% correct," only to make public statements to the contrary.

"I am not happy, but I have a smile on my face because the truth in all of this is going to emerge," Woodward said. "There's too much evidence, too many witnesses."

Bob Woodward says a White House official told him his new, explosive book about the Trump administration is "1,000% true," only to say the opposite in public.

Woodward was interviewed on Tuesday's episode of the New York Times podcast "The Daily" about his new book, "Fear: Trump in the White House."

He said officials in President Donald Trump's White House were particularly secretive and wouldn't tell the truth unless they were granted anonymity.

"After the information in 'Fear' started breaking last week, one key person who is in office called me and said, 'Everyone knows what you said here is true — it's 1,000% correct,'" he said.

"And then this person has said some public things that contradict that. And I am not happy, but I have a smile on my face because the truth in all of this is going to emerge. There's too much evidence, too many witnesses."

On the podcast, the Times Washington correspondent Michael Schmidt asked Woodward whether he thought journalists covering Trump's White House were using more anonymous sources compared with previous administrations.

Woodward, who has covered every administration since his groundbreaking coverage of President Richard Nixon and the Watergate scandal, said yes.

"Things are more hidden," Woodward said. He said anonymous sources "were the only way to get at the truth."

Woodward also explained why he used unnamed sources in his new book, saying he felt there was "no alternative" if he wanted to get the truth.

Trump has mentioned the anonymous sources in his attacks on the book, which he has called a work of fiction. "The Woodward book is a Joke - just another assault against me, in a barrage of assaults, using now disproven unnamed and anonymous sources," he tweeted.