The seventh debate involving Democratic hopefuls for this year's presidential election kicked off with blunt exchanges over the United States' policy in the Middle East.

The candidates were asked individually why they were qualified to be the country's commander in chief.

US Senator Bernie Sanders of Vermont brought up the 2003 Iraq War, saying he had helped to lead the effort against it.

He called the Iraq invasion "the worst foreign policy blunder in the modern history of this country".

"Joe (Biden) and I listened to what (then vice-president) Dick Cheney and (then president) George Bush and (then secretary of defense Donald) Rumsfeld had to say," Sanders said. "I thought they were lying. I didn't believe them for a moment. I took to the floor (of the House of Representatives). I did everything I could to prevent that war. Joe saw it differently," he said.

Former US vice-president Biden, who voted in 2002 to authorize military action in Iraq, said: "It was a mistake to trust that they (the George W. Bush administration) weren't going to go to war. They said they were just going to get inspectors (to search for weapons of mass destruction). The world, in fact, voted to send inspectors in and they still went to war."

Biden said that when Barack Obama chose him as his running mate for the 2008 presidential election, Obama subsequently relied on him to draw down the US troop presence in Iraq.

Biden criticized the Trump administration on its approach to Iran. "We're in a situation where our allies in Europe are making a comparison between the United States and Iran, saying both ought to stand down, making a moral equivalence. We have lost our standing in the region."

US Senator Amy Klobuchar, of Minnesota, said: "Donald Trump is taking us pell mell toward another war", in reference to the confrontation with Iran after the US killed Iranian general Qasem Soleimani in a drone strike at the Baghdad airport on Jan 3.

Iraq's Parliament recently passed a nonbinding resolution calling for US troops to leave the country.

US Senator Elizabeth Warren, of Massachusetts, said: "We need to get our combat troops out (of Iraq)".

Pete Buttigieg, an ex-mayor of South Bend in Indiana and former US naval officer who served in Afghanistan, said: "The president's actually sending more (troops)."

Biden was cautious about pulling all US forces out of Iraq because of the threat of terrorism in the region: "There's no way you negotiate with terrorists. They're going to come to us. They've come to us before, and they're going to come to us again."

Buttigieg said that if US troops can summon the courage to fight, the Congress needs to do the same to prevent military action without congressional approval.

Afghan war

Of the nearly two-decade US military presence in Afghanistan, Warren said she has listened to "one general after another who says we've just turned the corner. ... We've turned the corner so many times we're going in circles. ... Time to get our combat troops home."

Buttigieg said that the Trump administration, "by gutting the (2015) Iran nuclear deal", had made the region more dangerous.

Billionaire environmentalist Tom Steyer favored economic sanctions on Iran as opposed to military action, and said "the US needs to look beyond being the world's policeman".

Sanders gravitated to a key issue of his, saying"87 million people have no healthcare or are underinsured". He said there are "500,000 people sleeping out on the streets tonight".

The debate at Drake University in Des Moines, Iowa, hosted by broadcaster CNN, featured the smallest field yet as the qualifying criteria was more stringent.

Tuesday's was the last debate before the Feb 3 Iowa caucuses, the first opportunity to gauge how voters stand on the candidates.