Joe Biden, the presumptive Democratic presidential nominee, emerged from more than two months seclusion Monday, wearing a black face mask during a visit to lay a wreath on the day the United States honors its war dead.

"Thank you for your service," the former vice president said to a well-wisher after paying his respects at the Veterans Memorial Park in New Castle, Delaware.

Biden's last public appearance was March 15 when he faced off against his former Democratic rival Bernie Sanders for a debate in a television studio held with no live audience.

Since then the 77-year-old has remained in isolation at his home in Wilmington, Delaware in keeping with recommended measures to protect the elderly and prevent the spread of COVID-19.

US President Joe Biden wore a black mask covering his mouth and nose at Monday's Memorial Day wreath-laying, in contrast with his rival for the US presidency, Donald Trump who has yet to wear a mask in public despite guidance to the contrary from his own health authorities.

Biden and his wife Jill placed a wreath of white flowers at the memorial park and observed a moment of silence during the brief ceremony before returning to their motorcade.

Asked by a reporter if he had a message for the country, Biden's reply was difficult to hear.

"Never forget the sacrifices that these men and women made. Never, ever, forget," he said through his face mask.

The restrictions imposed by the coronavirus pandemic have created big obstacles for Biden's campaign, at a time when Trump has enjoyed high visibility through near daily White House pandemic briefings.

Biden abruptly cancelled a rally in Ohio March 10, replacing it with a speech to a half empty hall in Philadelphia that evening, followed by a news conference at a Wilmington hotel March 12.

Although he has celebrated primary victories while in confinement, he has been forced to conduct his campaign online for more than two months from a television studio installed at his home.

According to the candidate, he and his wife have left their home only for occasional walks or bike rides.