As Democratic presidential contenders sparred at their last debate of the year, frontrunner Hillary Clinton suggested that her Republican rival Donald Trump's rhetoric against Muslims was becoming a potent and powerful tool for Isis.

"He is becoming Isis' best recruiter. They are going to people showing videos of Donald Trump insulting Islam and Muslims in order to recruit more radical jihadists," she said during Saturday's Democratic debate, hosted by ABC News.

When asked about her emphasis on gun control in the wake of the San Bernardino terror attacks by a Pakistani-origin couple, Clinton again hit out at Trump saying the real-estate mogul was pushing the wrong narrative in the fight against the Isis (Islamic State of Iraq and Syria) militant group.

"I worry greatly that the rhetoric coming from the Republicans, particularly Donald Trump, is sending a message to Muslims here in the United States and literally around the world," she said.

That message seemed to be "that there is a clash of civilisations, that there is some kind of western plot or even war against Islam, which then I believe fans the flames of radicalisation", said Clinton.

Clinton's closest rival, self-styled democratic socialist Bernie Sanders, too attacked Trump, but for supporting low wages. "He thinks low wages are a good idea," he said.

Former Maryland governor Martin O'Malley, whose campaign has not gained much traction, said the US "must never surrender our American values to racists, must never surrender them to the fascist pleas of billionaires with big mouths".

The three candidates also clashed over gun rights, with Clinton saying more citizens purchasing firearms wouldn't help matters and Sanders focusing on a search for "consensus" on gun regulations.

"Guns in and of themselves, in my opinion, will not make Americans safer. We lose 33,000 a year already to gun violence. Arming more people -- to do what-- is not the appropriate response to terrorism," Clinton said.

Sanders disagreed, saying: "It's a divided country on guns. But there is a broad consensus on gun safety regulation".

He called for background checks for potential gun owners and the closure of loopholes that allow easier purchases at gun shows.

O'Malley attacked both, saying: "Secretary Clinton changes her position on this every election year, it seems. What we need on this issue is not more polls. We need more principle."