Former US vice-president Joe Biden has again said he is confident of victory as he inches closer to beating Donald Trump after Tuesday's US presidential election.

The Democratic challenger now has 253 of the 270 Electoral College votes needed to clinch the White House under the state-by-state US voting system.

Biden leads vote counts in the battlegrounds of Georgia, Nevada, Pennsylvania and Arizona.

A Biden win would see Trump leave office in January after four years.

As counting enters its fifth day, it is still unclear when the contest will end.

Officials are tallying up record numbers of postal votes due to the coronavirus pandemic, causing the longest delay to a presidential election result in 20 years.

What did Biden say?

"We're going to win this race," Biden told supporters in Wilmington, Delaware, on Friday night, striking a confident tone. He was joined by his running mate, California Senator Kamala Harris.

He said he was on track to win more than 300 Electoral College votes and pointed out that more people had voted for his campaign - over 74 million people - than any US presidential candidate in history.

Biden said Americans had given him the mandate to tackle the coronavirus pandemic, the struggling economy, climate change and systemic racism. On Friday for the third straight day, the US set a fresh record for new Covid-19 cases, with more than 127,000 infections.

The Democrat - presenting himself as the candidate of unity after a bitterly fought campaign - said it was time to "get the vitriol out of our politics" and "be civil to one another".

"We may be opponents but we're not enemies, we're Americans," said Biden, who did not mention his Republican opponent, Trump.

Biden's appearance had originally been planned as a victory speech, but he opted instead to give a general update on the state of the race as US TV networks cautiously held off declaring him the winner.

The Democrat said he hoped to address the nation again on Saturday.

What happens next?

Biden - who ran twice previously for the White House, in 1988 and 2008, without success - would be the oldest president ever inaugurated at 78.

If he is declared the victor this weekend, his team is expected to begin its transition process on Monday. The New York Times reports Biden could announce the first senior officials in his potential administration as early as next week.

The Secret Service has sent reinforcements to Delaware to beef up Biden's security detail. The Federal Aviation Administration has restricted flights over Wilmington's airspace.

However, there is no indication Trump will concede to his opponent in the short term.