US President Joe Biden has been interviewed as part of a probe into his handling of classified documents after he left the vice-presidency in 2017.

Mr Biden met voluntarily with Special Counsel Robert Hur at the White House over two days, officials said.

Mr Hur was appointed after a separate investigation was launched into secret documents found at Donald Trump's home.

Mr Biden has not been charged with any crime. It is not unusual for presidents to be interviewed by investigators.

Ian Sams, a spokesman for Mr Biden, said in a statement that the interview was conducted over the course of Sunday and Monday.

It was personally carried out by Mr Hur, who was chosen by US Attorney General Merrick Garland to lead the investigation. He added that the interview had "concluded" by the end of Monday.

"As we have said from the beginning, the President and the White House are cooperating with this investigation, and as it has been appropriate, we have provided relevant updates publicly, being as transparent as we can consistent with protecting and preserving the integrity of the investigation," Mr Sams told reporters.

"We would refer other questions to the Justice Department at this time."

The documents were discovered by aides to Mr Biden in an office he used after departing the vice-presidency, and before he launched his bid for the White House.

Watch: How much do you know about classified documents?

The first batch of classified documents had been found on 2 November at the Penn Biden Center, a think-tank the president founded in Washington DC.

A second batch of records was found on 20 December in the garage at his Wilmington home, while another document was found in a storage space at the house on 12 January, his lawyers said at the time.

After finding the documents, the president said his team immediately turned them over to the National Archives and the Justice Department. It is not clear why Mr Biden had kept them.

Under the Presidential Records Act, White House records are supposed to go to the National Archives once an administration ends, where they can be stored securely.

Mr Biden has previously said that he did not know the documents were there.

Authorities have not said what the files relate to. Classified documents can cover a range of issues, from the relatively mundane to the extremely secretive.

But sources have previously told CNN and the New York Times that the first set of 10 classified documents included briefing materials on foreign countries - including Ukraine, Iran and the United Kingdom - from Mr Biden's time as vice-president.

And according to CNN, these documents were mixed in with other non-classified papers, including details about the funeral of Beau Biden, the president's son, who died in 2015.

It also reported that some of the documents were labelled as top secret, the highest level of classification.

The interview took place amid dramatic tensions in the Middle East and had been scheduled months earlier, the New York Times reported, citing an unnamed White House official.

The conclusion of the interview could be a sign that the investigation may be nearing its end. Mr Hur's team has yet to decide whether to bring charges against Mr Biden.

In a separate and wider-range investigation, Special Counsel Jack Smith is bringing charges against Mr Trump for allegedly retaining highly-sensitive government documents at his Florida home and golf resort after leaving office.

Mr Trump is facing seven charges related to the documents, including wilfully retaining national defence secrets in violation of the Espionage Act and attempting to obstruct the official investigation.

He has repeated claimed it was his right to keep the documents, and is set to go on trial in Miami in May.