WASHINGTON - The FBI interviewed presumptive Democratic presidential nominee Hillary Clinton on Saturday as part of the investigation into her use of a private email server for official emails while she served as secretary of state, campaign spokesman Nick Merrill said.

"She is pleased to have had the opportunity to assist the Department of Justice in bringing this review to a conclusion,” Merrill said in a statement, describing the meeting as voluntary.

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

The 3 and 1/2 hours interview was conducted at FBI Headquarters in downtown Washington.

The Justice Department wants to finish its probe and recommend whether charges should be filed prior to the start of the Republican and Democratic national conventions, which start the week of July 18, ABC News reported.

On Friday, Attorney General Loretta Lynch said she will accept the decision of career prosecutors, investigators and FBI Director James Comey on whether to bring criminal charges in the investigation.

The unusual public announcement during an event in Aspen, Colorado, came as the attorney general faced a storm of criticism related to an awkward encounter with former president Bill Clinton after the two crossed paths earlier this week at Phoenix's Sky Harbor International Airport.

"This case will be resolved by the team that has been working on it from the beginning,” Lynch said, acknowledging that the meeting with Bill Clinton "cast a shadow” over the ongoing inquiry.

Meanwhile, presumptive Republican nominee Donald Trump was spending the weekend vetting potential running mates, including former House Speaker Newt Gingrich, New Jersey Governor Chris Christie, and Indiana Governor Mike Pence.

Trump was incredulous over Bill Clinton's meeting with Lynch, saying in a speech Friday, "He opened up a Pandora's box."

AFP adds: Clinton, aiming to become the nation's first female commander-in-chief, has apologised 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.

The FBI interview came amid revelations that US Attorney General Loretta Lynch held an impromptu meeting with Clinton's husband, former president Bill Clinton, at the airport in Phoenix, Arizona this week. The exchange touched off a political firestorm.

On Friday, Lynch vowed to respect the decisions of the FBI and prosecutors on whether to charge Hillary Clinton.