Sunnis believe that the security forces are carrying out sectarian reprisals, in part to combat the insurgency, but also in revenge for years of repression at the hands of Saddam Hussein's government.

Ayad Allawi, a prominent Iraqi politician who is close to the Sunni community, charged in an interview published Sunday in The London Observer that the Iraqi government -- and the Ministry of Interior in particular -- was condoning torture and running death squads.

The allegations raise the possibility of the war being fought here by a set of far messier rules, as the Americans push more responsibility for fighting it onto the Iraqis. One worry, expressed repeatedly by Americans and Iraqis here, is that an abrupt pullout of American troops could clear the way for a sectarian war.

One Sunni group taking testimony from families in Baghdad said it had documented the death or disappearance of 700 Sunni civilians in the past four months.

An investigator for the human rights organization said it had not been able to determine the number of executions carried out by the Iraqi security forces. So far, the investigator said, the evidence was anecdotal, but substantial.

"There is no question that bodies are turning up," said the investigator, who agreed to speak on the condition of anonymity, citing safety concerns. "Quite a few have been handcuffed and shot in the back of the head."

As an example, the human rights investigator said that the group had been able to verify that a number of Sunni men taken from the Baghdad neighborhood of Huriya and shot to death last August. Relatives of the dead told the group that more than 30 men had been taken from their homes by the Iraqi police in what appeared to be a roundup of Sunni males.