Violence across Iraq has declined and militias are making more efforts to disguise their grisly handiwork -- burying bodies in shallow graves and dumping them in sewers.

They are no longer boldly advertising their slaughters or leaving bodies in the plain view, Newsweek magazine reports in its upcoming issue.

Robert Lamburne, the director of forensic services at the British Embassy, has spoken to dozens of Iraqi policemen and examined bodies -- relatively fresh -- from one of several graves uncovered recently. His judgment: "There's less killing, but there's more concealment".

The magazine says about 600 Iraqi civilians were killed this November against 3000 in December 2006 -- a sharp decline.

But it also says the problem and the reason no one from US commander Gen David Petraeus to down is declaring victory yet is that those statistics do not tell the whole story.

In the past two months, Newsweek says, more than half a dozen mass graves have been found in Iraq, at least half of them in Baghdad.

At one site discovered late in November, in a yard in Baghdad's Saydiya neighbourhood, bodies and their severed heads were buried in two separate holes, it quotes a source at the Ministry of Interior as saying.

16 more bodies were found buried in a ditch north of Baghdad last Thursday. Dumping bodies, the magazine says, is nothing new in Iraq: Saddam Hussein filled mass graves with tens of thousands of Iraqis.

But in the heat of the civil war, militias boldly advertised their slaughter. Bodies -- headless, burned, slashed open and perforated with drill holes -- were left in plain sight as a message to others.

Now, with most Baghdad neighbourhoods dominated by one sect or the other, the death squads can afford to be more subtle in their killing, the report adds.

"Many militia groups just make people disappear," the magazine quotes Hicham Hassan, a spokesman for the International Committee of the Red Cross, as saying.

The strategy, it says, also reflects some positive developments in Baghdad. With many more US and Iraqi troops out on the streets, killers cannot be as brazen as before.

Most of the recent graves, says Newsweek, have been found in places where security has improved. In provinces like Anbar and Diyala, US troops have discovered graves as they've pushed into territory once controlled by al Qaeda in Iraq.

In Baghdad, locals returning to their old neighbourhoods have pinpointed sites by the smell of rotting bodies, it adds.

"Militias may change their tactics and put in plans to counter our plans," Ministry of Defence spokesman Muhammad Askari told the magazine when asked about fighters' hiding bodies. "But they can't match our capabilities."

But the problem with Baghdad's new calm, Newsweek says, is that no one is sure how real it is.

No Iraqi ministry keeps overall track of missing people, it adds and quotes aid workers as saying that Iraqi civilians often turn to their neighbours or local organisations, or body hunters before asking the police for help.