WASHINGTON: The FBI interviewed Hillary Clinton on Saturday about her use of personal email while serving as secretary of state, an issue that has dogged her campaign to become America's first female president.

Questions over Clinton's use of a private account and homebrew server during her time as America's top diplomat have fueled voter concerns that she is not trustworthy.

An aide said the interview at FBI headquarters in Washington lasted about 3.5 hours, following previous such interviews with Clinton aides, including close confidante Huma Abedin.

It comes about three weeks before the Democratic National Convention is set to crown Clinton as the party's official White House nominee.

The interview signals that the FBI's protracted criminal investigation could be entering its final phase, with a long-awaited decision nearing.

"Secretary Clinton gave a voluntary interview this morning about her email arrangements while she was secretary," spokesman Nick Merrill said in a statement.

His use of the word "voluntary" indicates that Clinton was not subpoenaed for the interview.

"She is pleased to have had the opportunity to assist the Department of Justice in bringing this review to a conclusion," Merrill said.

"Out of respect for the investigative process, she will not comment further on her interview."

Clinton, aiming to become the nation's first female commander-in-chief, has apologized for exclusively using a private email account and her own server during her time as secretary of state from 2009 to 2013.

Opponents argue that this breached rules about protecting classified documents from cyber attack and may have amounted to a crime.

Her use of private email for official correspondence first came to light in 2015 during Republican-led congressional investigations into her handling of a militant attack on the US mission in Benghazi, Libya.

The assault in 2012 left the US ambassador and three other Americans dead.

Clinton turned over some 30,000 emails to State Department officials after she stepped down from the job three years ago.

But she also said she deleted more than 30,000 other emails that were of a personal nature and not related to her work as secretary of state.

A starkly critical report by the State Department's inspector general found she had not sought permission to conduct official business on her personal account.