The latest to die were four soldiers whose patrol vehicle was blown up by a b**** in southern Baghdad on Sunday.

In response to the news, US Vice-President Dick Cheney said he regretted every US casualty in Iraq.

Insurgent attacks and military operations left at least 47 people dead across Iraq on Sunday.

According to an Associated Press breakdown of the figure, 97% of deaths occurred after US President George W Bush declared on 1 May 2003 that major c****at was over.

Roadside b****s accounted for 44% of deaths last year and 55% to date in 2008, the agency adds.

In Sunday's other violence:

• Mortar and rocket fire killed at least 15 people in Baghdad's heavily fortified Green Zone
• A suicide b****er ploughed an explosives-laden tanker into an army base in Mosul, killing 13 Iraqi soldiers and injuring 40 people
• US troops killed 12 men in a raid east of Baquba, saying that six of them were found to have shaved their bodies, suggesting they had been preparing for suicide operations
• Gunmen travelling in three cars shot dead at least seven shoppers in a Baghdad market.

'Less visible'

Sunday's *lo**shed comes despite an overall reduction in violence since last June.

That was when the US deployed an extra 30,000 troops in violence-hit areas - the so-called "troop surge".

Stephen Biddle, a senior fellow for defence policy at the Council on Foreign Relations in Washington, said he doubted the 4,000 milestone would "have the impact that the 3,000 did" in December 2006.

"The conventional wisdom then was that things were going badly," he was quoted by Reuters as saying.

"Today, by contrast, the public's general perception of Iraq is less negative, and coverage for the last six months has tended to focus on the reduction in violence and US casualties.

"The war has also been much less visible."

US soldiers interviewed by AFP news agency in Iraq said they were saddened by the figure of 4,000 but argued the conflict was justified.

"Every one of those people signed up voluntarily and it's a shame that that happens, but tragedies do happen in war," said senior Airman Preston Reeves, 26.

Staff Sergeant Jonathan Criss, 37, said the US could not "just walk away and leave the Iraqi people".