US Democratic presidential candidates have clashed over who has been strongest on gun control in their third debate.

A dispute over a campaign data breach intensified the battle between front-runner Hillary Clinton and Bernie Sanders on Saturday.

Clinton, Sanders and a third candidate, former Maryland Governor Martin O’Malley, also debated the best way to defeat Islamic State militants and prevent “lone wolf” attacks like the one carried out by a radicalised Muslim couple who killed 14 people in a December 2 shooting spree in California.

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O’Malley, who is lagging in polls behind former Secretary of State Clinton and US Senator Sanders in the race for the 2016 Democratic presidential nomination, accused his opponents of adopting a more aggressive stance on gun control in the wake of a number of mass shootings this year culminating with the massacre in San Bernardino, California.

O’Malley charged that IS militants have advised recruits that the best way to get a weapon in the United States is at a gun show where rules are more lenient on the purchase of a firearm. This is a result, he said, of “flip-flopping” by Sanders and Clinton.

“Whoa, whoa, whoa, Martin,” Sanders said at the ABC debate.

“Let’s tell the truth, Martin,” Clinton said.

Sanders said he had lost an election in Vermont for a gun-control stance and Clinton said she had backed gun-control measures.

The Democrats’ third debate was marked by controversy from the start over a recent data breach of Clinton’s campaign voter files by a Sanders staffer, who was subsequently fired.

Sanders apologised to Clinton for the breach.

“Yes, I apologise,” he said, when asked about the controversy during the debate, but he renewed his criticism of the Democratic National Committee for freezing access to his own voter files until the issue was resolved late on Friday.

Clinton, whose campaign said Sanders made a number of breaches into Clinton computer files, accepted the apology and said it was time to move on.

Clinton, leading polls in the nomination fight, attacked Republican presidential front-runner Donald Trump several times, chiefly for his proposal to ban Muslims from entering the United States.

She said IS militants are showing videos of the billionaire tycoon talking about his proposed ban as a recruitment tool.