SOMERSWORTH, NEW HAMPSHIRE (REUTERS) - Former United States vice-president Joe Biden vowed on Wednesday (Feb 5) to go on fighting for the Democratic presidential nomination despite what he called "a gut punch" he took in Iowa's contest, where partial results showed the political veteran lagging in fourth place.

"I am not going to sugarcoat it: We took a gut punch in Iowa. The whole process was a gut punch," Mr Biden said in Somersworth, New Hampshire, where he was campaigning.

With about 71 per cent of precincts reporting from Monday's caucuses, Mr Biden was behind Mr Pete Buttigieg and US senators Bernie Sanders and Elizabeth Warren in the first nominating contest of the 2020 campaign culminating in a Nov 3 election.

Mr Biden, who bills himself as the most electable Democratic candidate to take on Republican President Donald Trump, led many national polls in the run-up to Iowa and has a host of high-profile endorsements.

But his campaign is now in trouble.

"This isn't the first time in my life that I've been knocked down," Mr Biden said.

"There are an awful lot of folks out there who wrote off this campaign... They've been trying to do that from the moment I entered the race. Well, I've got news for them: I'm not going anywhere," he said.

In an unusually direct address, Mr Biden took aim at Mr Sanders and at Mr Buttigieg, the former mayor of South Bend, Indiana, as he tried to recover ahead of the New Hampshire state primary next Tuesday.

Mr Biden, 77, said every Democrat running for the House of Representatives or Senate this year would have to carry the label "socialist" if Mr Sanders became the Democratic nominee. An independent, Mr Sanders calls himself a democratic socialist.

Mr Biden accused Mr Buttigieg, 38, of being insufficiently supportive of the achievements of the Obama administration and said it would be a risk for Democrats to nominate someone who had never been more than mayor of a small town. Mr Biden was former president Barack Obama's vice-president for eight years.

Mr Buttigieg held a narrow lead over Mr Sanders in the Iowa caucuses, according to partial results. Problems with an app used for vote counting delayed a final count. Ms Warren, 70, placed third.

Mr Buttigieg, who would be the first openly gay US president if elected, had 26.8 per cent of state delegate equivalents, the data traditionally reported to determine the winner. Mr Sanders had 25.2 per cent, Ms Warren was at 18.4 per cent and Mr Biden garnered 15.5 per cent.

Mr Sanders, 78, was ahead in the Iowa popular vote, which is not used to determine the delegates who will formally choose the nominee at the Democratic National Convention in July.

Mr Buttigieg has argued it is time for a new generation of leaders and that his lack of experience in Washington makes him an ideal candidate to break the partisan gridlock there.

But he might struggle to win in New Hampshire, where Mr Sanders leads in many opinion polls, and in the next Democratic primary in South Carolina on Feb 29 where Mr Biden expects to receive strong backing from the African-American vote.