Joe Biden’s team says it is “inevitable” he will be president as the Democrat’s lead extended over Donald Trump, with four key states to decide the outcome.

But Mr Trump’s side is also claiming it’s poised to snatch victory by as early as Friday evening (local time) as the Republicans lodged legal action in several states.

Counting continues in the four key states of Georgia, Arizona, Nevada and Pennsylvania in the knife-edge US election as Mr Trump demanded: “STOP THE COUNT” and “STOP THE FRAUD” on Twitter.

Analysts say Mr Biden’s path to winning 270 Electoral College Votes and the presidency could hinge on the battleground of Arizona where his team believes they could win by “tens of thousands of votes”.

If Mr Biden can hold a lead in Arizona, where he is in front by 68,000 votes, any other state coming in for him will likely push his campaign over the line, AAP reports.

The BBC has the electoral points tally at 253 to Mr Biden and 214 for Mr Trump.

“We do expect the margin to be closing (in Arizona). This is the trajectory. We will win by tens of thousands of votes but that margin will close,” said Biden campaign manager Jen O’Malley-Dillon.

• Read: Trump will do anything for victory but can his lawsuits change the outcome?

As of Friday morning (Australian time), Mr Biden’s advisers said the Democrats were also on track to claim Nevada where Mr Biden’s lead stretched from 7,647 ballots to more than 11,000.

Mr Biden holds 49.4 per cent of the Nevada vote, while Mr Trump holds 48.5 per cent, the BBC reports.

“We should see some ups and downs throughout the day … but at the end of the day, when all the votes are counted we are confident we will be ahead in Nevada,” said Ms O’Malley-Dillon.

In Georgia and Pennsylvania, Mr Trump’s narrow leads were shortening and Mr Biden’s team believed the remaining votes in Pennsylvania would continue to trend blue and close the gap.

Tweet from @realDonaldTrump

Mr Trump’s lead shrunk significantly in Pennsylvania from 600,000 votes at one stage to 115,000, with the postal votes trending for Mr Biden.

In Georgia, Mr Trump is ahead by about 12,000 votes but Mr Biden is inching closer, with less than 48,000 votes to count as of Friday morning (Australian time).

Mr Biden urged supporters to “stay patient and stay calm” as vote counting continues.

“Be patient, folks. Votes are being counted, and we feel good about where we are,” he tweeted.

Ms O’Malley Dillon said Mr Trump’s attempt to halt counting in some states and seek a recount in Wisconsin was a “flailing strategy” and an “attempt to distract and delay” from the results of the election.

Mr Trump has repeatedly demanded that election officials stop counting votes and has launched three lawsuits which have had little effect so far.

In a statement on Friday morning (Australian time), Mr Trump said: “If you count the legal votes, I easily win the election!

“If you count the illegal and late votes, they can steal the election from us.”

Tweet from @ebonybowden

Mr Trump wants to take the election to the Supreme Court for “all voting to stop”.

“STOP THE FRAUD!” he wrote on Twitter but the social network hid the message behind a warning that “some or all of the content shared in this Tweet is disputed and might be misleading about an election or other civic process”.

Twitter also censored Mr Trump’s post this morning about there being “plenty of proof” that “voter fraud” helped Mr Biden claim victory in “all of the recent Biden claimed States”.

His petition to have his voter fraud claims heard in the Supreme Court has been backed by Republicans in key battleground states.

In an attempt to remove mail-in ballots from the vote tally, Republicans have filed lawsuits in Pennsylvania, Michigan and Georgia, and have asked for a vote re-count in Wisconsin.

They say that mail-in ballots that arrived after the state deadlines are being counted anyway and that Republican poll watchers are being shut out.

It is unclear whether these lawsuits will have the desired effect of having some mail-in ballots disqualified.

-with AAP