Hillary Clinton told the FBI she did not recall all the briefings she received on handling sensitive information as she made the transition from her post as US secretary of state, due to a concussion suffered in 2012, according to a report.

The Federal Bureau of Investigation (FBI) released a summary of the 2 July interview it conducted with the Democratic presidential candidate, as well as other details of its investigation into her use of a private email server while heading the State Department.

Mrs Clinton, who is challenging Republican Donald Trump for the White House in the 8 November election, has been dogged by the fallout from her private email account for more than a year.

Republicans have repeatedly attacked Mrs Clinton over the issue, helping drive opinion polls that show many US voters doubt her trustworthiness.

The Democratic candidate has said that in hindsight she regretted using a private email system while secretary of state.

The report said: "Clinton said she received no instructions or direction regarding the preservation or production of records from (the) State (Department) during the transition out of her role as Secretary of State in 2013.

"However, in December of 2012, Clinton suffered a concussion and then around the New Year had a blood clot (in her head). Based on her doctor's advice, she could only work at State for a few hours a day and could not recall every briefing she received," the report said.

According to the report, Mrs Clinton told the FBI that she did not set up a private email server to sidestep the law requiring her to keep her business communications a matter of public record.

Mrs Clinton has claimed it was public knowledge to many State Department employees that she was using a private server because they received emails from her email domain.

But State Department employees interviewed by the FBI said many emails from Clinton appeared to be from "H" and did not show her private email domain.

The documents also show that Mrs Clinton contacted former secretary of state Colin Powell in 2009 to ask about his use of a personal BlackBerry phone.

In his reply to Mrs Clinton via email, Mr Powell told her to "be very careful" because the work-related emails she sent on her BlackBerry could become public record.

"I got around it all by not saying much and not using systems that captured the data," Powell said.