US Democratic presidential candidate Bernie Sanders has censured front-runner Hillary Clinton for her support of the invasion of Iraq in 2003.

The former secretary of state on Wednesday called on the Vermont senator to apologize to Sandy Hook victims because of his stance against holding gun manufacturers accountable for gun crimes.

In response, Sanders said that Clinton should apologize to the victims of the Iraq war, which she voted in favor of as a senator.

In March 2003, the United States and Britain invaded Iraq in blatant violation of international law and under the pretext of finding Weapons of Mass Destruction; but no such weapons were ever discovered in Iraq.

More than one million Iraqis were killed as the result of the US-led invasion, and subsequent occupation of the country, according to the California-based investigative organization Project Censored.

Sanders said in an interview last week that he doesn't believe that victims of gun violence should be able to sue the company that made the gun the shooter used to kill twenty children and six adults on December 14, 2012 at the Sandy Hook Elementary School in Newtown, Connecticut.

"In the same sense that if you're a gun dealer and you sell me a gun and I go out and I kill him," Sanders said. "Do I think that that gun dealer should be sued for selling me a legal product that he misused?"

"But I do believe that gun manufacturers and gun dealers should be able to be sued when they should know that guns are going into the hands of wrong people,” he added.

Sanders, whose campaign was boosted by landslide victories in Alaska, Washington, Hawaii and Wisconsin, is hoping to cut further into Clinton’s lead in her home state of New York, where voters will cast their ballots on April 19.

Sanders has long been critical of US foreign policy and was an early opponent of the wars in Iraq and Afghanistan. Clinton tends to be more hawkish and inclined to use military force.

George W. Bush (left) and Hillary Clinton watch a video at the Summit to Save Lives on Sept. 13, 2011, in Washington. (Getty Images)

In February, the 74-year-old politician denounced former President George W. Bush over misleading the public in order to justify the Iraq war, and Clinton over her vote in favor of the war.

“I didn’t believe them,” he said. “I voted against the war. My opponent, Secretary Clinton, voted for the war.”

The US war in Iraq cost American taxpayers $1.7 trillion with an additional $490 billion in benefits owed to war veterans, expenses that could grow to more than $6 trillion over the next four decades counting interest, according to a study called Costs of War Project by the Watson Institute for International Studies at Brown University.