DES MOINES (AFP) - Senator Bernie Sanders claimed victory in the Iowa caucuses on Monday (Feb 3), citing internal campaign data that placed him ahead of Mr Pete Buttigieg after a major delay to the release of official results.

The first vote of the US election season, Iowa's caucuses kick off the Democratic primary that will determine who challenges Mr Donald Trump in November.

Data released by the Sanders campaign, which said it represents results from nearly 40 per cent of precincts in Iowa, shows the leftist senator garnering 28.62 per cent of the vote, followed by moderate Buttigieg on 25.71 per cent.

"We recognise that this does not replace the full data from the Iowa Democratic Party, but we believe firmly that our supporters worked too hard for too long to have the results of that work delayed," Sanders advisor Jeff Weaver said.

The data collected by Sanders campaign volunteers showed progressive Senator Elizabeth Warren in third place, with 18.42 per cent of the vote, with former vice-president Joe Biden slipping to fourth place on 15.08 per cent.

The only other candidate polling above 10 per cent was Ms Amy Klobuchar, on 10.93.

Monday's contest turned sour after major hiccups in the reporting process prevented the release of official results.