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Sander's campaign released its own internal polling data, which show him receiving approximately 40 percent of the vote, according to The Independent.

The win could represent a dramatic last-minute turnaround for his campaign, whose success was under threat late last year with a string of poor performances in the polls as the senator lay in a Las Vegas hospital, recovering from a heart attack.

Pre-caucus polls had suggested Sanders entered the night with a narrow lead, but any of the top four candidates - Sanders, Biden, Warren and Buttigieg - were also poised to score a victory.

The Iowa Democratic Party said on Tuesday that delays in reporting the outcome were due to an earlier coding issue and that the party hoped to release results "as soon as possible".

President Trump tweeted that the delay was an "unmitigated disaster" for the Democratcs, suggesting it was proof they could not be trusted in office.

"The only person that can claim a very big victory in Iowa last night is 'Trump'," he wrote.

On Monday, voters in Iowa turned out en masse to over 1,600 schools, libraries and churches to decide their chosen candidate for the upcoming US elections in November.

In a caucus, people attending a meeting before voting on a candidate, either via a head count or a show of hands.

Trump won the Republican contest with a sweeping majority, but a winner was not declared in the Democratic contest.

Critics wonder whether the Iowa caucuses, often a complex set of political meetings in a state whose population is predominantly white and aging, at odds with the current demographic of the Democratic Party, are the tradition of a by-gone era.

The party tried to accommodate critics this year by promising to report three different data points about voters' preferences, with the aim of improving transparency.

But the new system created new headaches.

State party spokeswoman Mandy McClure said the party had "found inconsistencies in the reporting of three sets of results", forcing officials to try to verify results with "underlying data" and the paper trail.

The trouble stemmed from issues with a new mobile app developed to report results to the party, with organisers reporting problems downloading the app and other glitches.

The Democratic candidates are trying to win the opening contest of the 2020 White House campaign to become the nominee who will challenge Trump.

Iowa awards just 41 of the 1991 delegates needed to clinch the nomination at the party's National Convention in June.

The state can offer much needed momentun to presidential hopefuls - the last four Democratic nominees have all won Iowa caucuses.

The race will now move on to the other three early-voting states of New Hampshire next week, then Nevada and South Carolina later in February.

Agencies contributed to this report.

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