Former US president George W. Bush says he remains "comfortable" with the decision to invade Iraq, even as a new spate of bloody violence hit the country and rocked politics in Baghdad.

Bush told ABC News in an interview marking the opening of his presidential library on Thursday that it was up to history to judge the invasion of Iraq in 2003, prompted by fears of weapons of mass destruction that were never found.

"I am comfortable in the decision-making process. I think the removal of Saddam Hussein was the right decision for not only our own security but for giving people a chance to live in a free society," Bush said.

"But history will ultimately decide that, and I won't be around to see it.

"As far as I'm concerned, the debate is over. I mean, I did what I did. And historians will ultimately judge those decisions."

The US invasion swiftly toppled Saddam Hussein but the mismanaged aftermath of the war led to US forces becoming embroiled in a prolonged insurgency.

Bush supporters say he gave Iraqis the chance to live in freedom, but detractors note that the country is still in the grip of widespread violence as hopes for a true democracy fade.

President Barack Obama, who finally succeeded in pulling US troops out of Iraq in late 2011, will attend the dedication of Bush's library in Dallas on Friday, with all the living ex-US presidents.

More than 4,400 US soldiers died in Iraq, many more were maimed and tens of thousands of civilians are believed to have died in violence which followed the US invasion and is still raging.

In the last two days alone, violence has killed 125 people in the Iraq, most of them in clashes and attacks involving security forces, protesters and their supporters, officials said.

At least 268 people have been wounded, 194 of them in protest-related unrest, which prompted two Sunni ministers to quit and has sent tensions in the country soaring.