Pete Buttigieg seized a shock lead Tuesday in Iowa's debut US presidential nomination vote, according to partial Democratic Party results that placed Bernie Sanders in close second and national frontrunner Joe Biden a distant fourth.

The first wave of results, following a delay due to technical issues in Monday night's Iowa caucuses, gives a major boost to the campaign of Buttigieg, a 38-year-old former mayor of South Bend, Indiana who has trailed the frontrunners for months but surged at the right time.

And it marked a disappointing start for Biden, who has repeatedly stressed on the campaign trail that he is the candidate best-positioned to defeat President Donald Trump in November's election.

With 62 percent of precincts reporting, Buttigieg leads with 26.9 percent of what Iowa calls the state delegate equivalent, the all-important metric that determines the allocation of delegates who choose the Democratic nominee at the party's national convention.

Sanders, the senator running as a self-declared democratic socialist and who had led the polling in Iowa, was closely behind at 25.1 percent, with fellow progressive Senator Elizabeth Warren at 18.3 percent

Biden, the best-known moderate in the race, was at 15.6 percent, narrowly ahead of Senator Amy Klobuchar, also a centrist, with 12.6 percent.

Democratic presidential candidate Pete Buttigieg, a gay millennial small-city mayor who was unknown nationally just a year earlier, hailed the "astonishing" early Iowa caucus results
Democratic presidential candidate Pete Buttigieg, a gay millennial small-city mayor who was unknown nationally just a year earlier, hailed the "astonishing" early Iowa caucus results AFP / Joseph PREZIOSO

Like other candidates, Buttigieg has already shifted his attention to New Hampshire, the state which votes next, on February 11.

Addressing a crowd of supporters in the town of Laconia, Buttigieg hailed the "astonishing" early results for a gay millennial small-city mayor who was unknown nationally just one year ago.

"No matter what happens next, this much is undeniable, that fact represents an astonishing victory for this campaign, this candidacy and this vision that you all have been a part of," he said.

Iowa's results were posted about 21 hours after the opening of caucuses that kickstart the race, in a vote that was marred by what the party called reporting "inconsistencies" due to a "coding error."

In Iowa's quirky system, caucus-goers commit to their candidate of choice, but if that candidate fails to reach the viability threshold of 15 percent, his or her supporters can re-align with a more viable hopeful.

In the realignment, Buttigieg appeared to be particularly successful, drawing support from many voters who saw him as their favorite choice after their failed candidate.