Advertising spending on the US presidential primaries has surpassed an "unprecedented" $1 billion, half of which came from the coffers of billionaire Democratic White House hopeful Michael Bloomberg, a tracking firm said Friday.

The former New York mayor, who was the ninth richest man in the world last year, has spent $539 million on television and radio spots, as well as digital ads, since launching his run on November 25, Advertising Analytics said.

With four months still to go before the Democratic convention, the spending has blown past the previous record of $338.2 million set by President Barack Obama during his 2012 re-election campaign.

Bloomberg is dipping into his personal fortune of some $60 billion, earning him sharp criticism from competitors who say he is trying to buy the election.

Tom Steyer, another billionaire competing to be the Democratic candidate to take on Donald Trump in the November election, has spent $186.1 million on advertising so far, Advertising Analytics said.

Senator Bernie Sanders, the current Democratic front-runner, is the top ad spender among non-billionaire candidates, thanks to his "prodigious fundraising ability," the firm said. He has been able to run television ads in key markets weeks ahead of his competitors, spending a total of $48.6 million so far.

By comparison, former vice president Joe Biden has spent a relatively anemic $13.9 million.

In total, the Democratic candidates combined have spent $969 million, while Republicans have spent $67.9 million in advertising for their party primary, though the nomination of incumbent President Donald Trump is only a formality.