The bodies of 6,000 people, most of whom died violently, have been received by Baghdad's main mortuary so far this year, health ministry figures show.
Mortuaries have become a focal point for families seeking loved ones
The number has risen every month, to 1,400 in May. The majority are believed to be victims of sectarian killings.
But observers say the real death toll could be much higher.
Meanwhile police said nine severed heads were found near Baquba to the north of Baghdad - days after a similar discovery there.
In another development, Iraq's prime minister says he plans to release 2,500 prisoners.
Nouri Maliki said the move, starting on Wednesday, is a gesture of "national reconciliation". Most of those to be released are Sunni Arabs, Iraqi officials say.
MORTUARY'S MONTHLY TOLL
Mr Maliki hopes that by announcing such a large release in one go it will help win over more members of the Sunni community and undermine support for the insurgency, says our correspondent.
In other developments:
- Five people are killed and more than 12 wounded when a car bomb explodes near to where a funeral is being held in south-western Baghdad
- The Iraqi army takes over from the US military in a part of the restive Anbar province
- Italy's Prime Minister Romano Prodi says the attack on its military that killed a soldier on Monday will not change Italy's timetable of withdrawing from Iraq
- Three British soldiers are cleared at a court martial in London of killing a 15-year-old Iraqi boy who drowned in a canal in Basra three years ago
The 2006 death toll at the Baghdad mortuary appeared in two local newspapers and was confirmed to the BBC by officials who asked to remain anonymous because the issue is so sensitive.
One reason for the sensitivity is that government officials fear more detailed information on these killings could further enflame sectarian tensions, the BBC's Andrew North in Baghdad says.
Tuesday's gruesome find followed a similar discovery on Saturday
But no-one believes these are the true figures from the violence in and around Baghdad as many bodies are not taken to the morgue, or are never found, he adds.
The nine severed heads had been wrapped in plastic bags and left in a fruit box by the side of a road near Baquba, 60km (35 miles) north-east of Baghdad, police said.
Some of the heads were blindfolded and already decomposing, suggesting that the killings had taken place a few days before, they added.
There is no word on the identity of the victims.
The heads of eight people were found the same area on Saturday - one of the dead was identified as a local Sunni preacher.
Baquba lies in a mixed Sunni Arab and Shia province where attacks against civilians have been common.
Prime Minister Maliki said the prisoners to be released would not include supporters of the ousted president, Saddam Hussein, or anyone who had Iraqi blood on their hands.
He said they would be released from US-run detention centres and Iraqi prisons.
"We hope they will abide by not violently objecting to the political process. This is a strong move which will encourage others," Mr Maliki said.
Iraq's Sunni Vice-President, Tariq al-Hashemi, was closely involved in deciding who should be set free.