Democrat Joe Biden said Thursday he has "no doubt" he will defeat President Donald Trump and be declared winner of the US election, insisting that voters remain patient and that the result will be known "very soon".

"We continue to feel very good about where things stand. We have no doubt that when the count is finished, Senator (Kamala) Harris and I will be declared the winners," Biden told reporters in his hometown of Wilmington, Delaware.

Biden, 77, is leading Trump in the race for the 270 electoral votes that will put one of them over the top, with the Democrat's campaign asserting they believe he has enough votes to win in key battleground states that remain undecided, like Pennsylvania. 

Biden gets more security as he edges toward win: report

The US Secret Service has increased its protective bubble around Joe Biden as chances increased that he will be the next US president, the Washington Post reported Friday.

The Secret Service sent an extra squad of agents to Biden's campaign headquarters in Wilmington, Delaware as expectations rose that the Democratic candidate would be able to declare victory over President Donald Trump as early as Friday, the Post reported.

The Secret Service, an agency under the Department of Homeland Security, is in charge or protecting the White House and senior government officials, visiting high officials, and others.

It had already deployed some agents to protect Biden around early July after he triumphed in the Democratic Party's presidential primaries.

As a former vice president, Biden could have requested Secret Service protection before then, but reportedly did not.

If Biden becomes president-elect, Secret Service protection is expected to ramp up to a higher level.