Security in Iraq is improving despite a wave of car bombings that include the worst single attack since the US-led invasion in 2003, General Raymond Odierno, the number two leader of US forces in Iraq said Friday.

Odierno said that the US "surge" troop hike launched in January is "beginning to pay off" despite the massive attack in northern Iraq which killed more than 400.

According to Odierno, such an attack in 2006 "might have triggered a spiral of revenge killings, but, today, such horrific events actually unite Iraqis of different ethnicities and confessions in their outrage."

"Yes, we continue to face setbacks here in Iraq," Odierno acknowledged, "but overall we continue to make steady progress."

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Four suicide bombers packed two tonnes of explosives into their trucks and attacked members of the ancient Yazidi religious sect in the deadliest incident since the war began.

"Despite this attack, security across Iraq is generally improving," said Odierno, speaking via teleconference from Iraq.

"Al-Qaeda is forced to undertake its spectacular events in more remote parts of the country, rather than in the capital. And we no longer see the cycle of sectarian revenge that plagued Iraq last year," he said.

Odierno's report came ahead of a much-expected and potentially pivotal review of the Iraq situation in September to be presented to Congress.

War commander General David Petraeus and US ambassador to Baghdad Ryan Crocker are to make both public and closed-testimony presentations on the effects of the surge, which added some 30,000 troops to those already in Iraq in a new effort to quell violence and defeat insurgents.

Both the administration of President George W. Bush and US opponents of the war see the September report as crucial to settling the debate over whether to begin withdrawing US troops from the country.

Odierno argued that the surge is working. "The effects of our surge operations and reconciliation efforts are beginning to pay off. Total attacks are on a month-long decline and are at the lowest level since August of 2006," he said.

"Attacks against civilians are at a six-month low. IED (improvised explosive device) attacks are at a two-month decline and have a 45 percent found-and-cleared rate. Civilian murders in Baghdad are down over 51 percent, reaching their lowest level since just before the Golden Mosque in Samarra was bombed in February of 2006."

Odierno said the constant pressure of the now 155,000 US troops in the country on "extremists" and alleged Al-Qaeda operatives and the "depletion of their leadership" through captures and killing has forced them out of population centers and left them constantly seeking new places to operate.

"We'll continue to aggressively target their shrinking areas of influence," he said.

He also said Iraqis themselves increasingly recognize coalition and Iraqi forces as crucial for security and are helping in the fight.

"Today, the Iraqis feel ...the appalling nature of this brutality -- and it galvanizes their rejection of Al-Qaeda and other extremist elements .... They are pointing out extremist leaders, identifying caches and IEDs, and asking to be a part of the legitimate Iraqi security force."

Odierno, though, emphasized the need for the Iraqi government to be able to manage security efforts -- an issue that will be a key in the September review.

"We understand that our recent tactical successes will only add up if Iraqis take advantage of them, and ultimately, the government of Iraq is a key to progress.

"It's imperative we continue to press on all fronts, diplomatic, political, economic, and governance in addition to our security efforts."