Assassins have shot dead a former senior leader of Al-Qaeda in Iraq who defected from the extremist network and sided with US forces, officials said on Wednesday.

The murder of Mullah Nadim al-Juburi, 34, came after he made remarks on Iraqi television of how the organisation's front group, the Islamic State of Iraq (ISI), had made key changes to its leadership.

"We feared such an incident would occur," said Amr Khuzaie, Prime Minister Nuri al-Maliki's adviser on national reconciliation, told AFP.

"Mullah Nadhim al-Juburi and others took the risk of being killed, but that will not prevent us from continuing to work with anyone who wants to rebuild the country."

Juburi, who worked in the national reconciliation commission, was killed on Tuesday night while driving in the west Baghdad neighbourhood of Mansur, police in the capital said.

A vehicle in front of him slowed down, forcing his car also to slow while a second vehicle pulled up alongside his and gunmen opened fire using silenced weapons.

"He worked with us (in the national reconciliation commission) after becoming aware of the crimes of Al-Qaeda," said Khuzaie, who blamed the ISI and supporters of executed dictator Saddam Hussein's Baath Party for Juburi's murder.

Juburi sparked the ire of extremists last week when he appeared on television to give details about changes to the ISI's operational structure in the leadership and in south Iraq, and provided the names of those in charge.

On the Honein jihadist Internet forum, several messages on Sunday had warned that Juburi's "days are numbered."

Juburi was the leader of ISI's fighters in Duluiyah, a predominantly Sunni town 90 kilometres (55 miles) north of Baghdad, before cutting ties with the group in May 2008 and siding with US forces.

He then became a leader of the Sahwa, comprised of Sunni tribal militias that turned away from Al-Qaeda from late 2006 onwards, helping turn the tide of Iraq's violent insurgency.

US says Iraq killings were 'tragic' for both sides
Washington (AFP) Jan 25, 2012 - The US State Department said Wednesday that the killings of 24 unarmed Iraqi civilians in Haditha in 2005 were "tragic" for both countries, but the US military had drawn valuable lessons from them.

State Department spokeswoman Victoria Nuland declined to comment on Tuesday's controversial verdict in the military trial of a Marine squad leader who accepted a plea deal under which he will serve no time in prison.

However, Nuland told reporters the events in Haditha "were tragic for both the United States and for Iraq.

"In the aftermath of this incident, the Department of Defense conducted a complete and comprehensive review of all of our rules of engagement for our soldiers in Iraq," Nuland said.

It also undertook a review of "training, other procedures to protect civilians during armed conflict," she said.

And the Pentagon "learned a huge number of lessons from this Haditha incident and was able to make changes to the way we do business as a result," she said.

"So (it was) very much a tragedy, something that we learned from," she said.

Staff Sergeant Frank Wuterich, who led an eight-man squad whose other members have all been let off, was sentenced to 90 days confinement but will not serve the term, under a deal with prosecutors.

Wuterich, who admitted to one count of negligent dereliction of duty, also was demoted to the rank of private.

The Baghdad government vowed Wednesday to take legal action following the verdict, which was widely panned in Iraq as being too light.

It closed a case that fueled anger and highlighted why the Iraqi authorities demanded that Americans be subject to local laws in failed talks to extend the US military presence in the country beyond 2011.

Wuterich himself voiced sorrow Tuesday, but insisted he was not a "cold-blooded baby killer."

Addressing three surviving family members of those killed in Haditha, he said: "Words cannot express my sorrow for the loss of your loved ones. I know there is nothing I can say to ease your pain.

"I wish to assure you that on that day, it was never my intention to harm you or your families. I know that you are the real victims of November 19, 2005," he said, reading from a prepared statement.