THE result of the US presidential election is too close to call, and the final outcome might not be known for several days.

Donald Trump and his rival Joe Biden are neck and neck in key swing states.

Republican Trump claimed to have won and vowed to launch a Supreme Court challenge, alleging fraud, while Democrat Biden said he was "on track" to victory.A total of 270 Electoral College votes are needed to become president, and mid-morning on Wednesday, Biden had 224 and Trump 213.

Millions of votes remain uncounted, while more than 100 million people cast their ballots in early voting before election day on Tuesday, setting the US on course for its highest turnout in a century.

Trump is projected to have held the must-win state of Florida - a major boost to his re-election bid, but Biden could snatch Arizona.

A loss for Mr Trump in that once reliably Republican state would be a potentially serious setback.

Other key states such as Georgia, Pennsylvania, Wisconsin, Michigan and North Carolina are hard to call.