Tony Blair has come under fire from MPs over his handling of claims that Iraqis have been mistreated by British troops.
Mr Blair said anyone responsible for abuse would be punished
At his weekly question time, he insisted ministers had not been slow to act on the claims from the Red Cross and Amnesty International.
But he accepted the past few days had been "immensely damaging".
Tory leader Michael Howard questioned why ministers had not seen until this week February's Red Cross report featuring "devastating" allegations.
As he tried to raise the pressure on the government, Mr Howard said: "A devastating Red Cross report is presented to the government in February, the armed forces minister says he has never seen it.
"The defence secretary says he wouldn't have expected to see it, the foreign secretary says he should have seen it, but didn't and the prime minister says he knew nothing about it.
"How can the people of this country have confidence in this prime minister and his government?"
Liberal Democrat leader Charles Kennedy asked what action had been taken as a result of the Red Cross reports in the past year.
Mr Blair insisted the organisation had found conditions at British-run facilities were now "fairly good".
The prime minister also paid tribute to the "vast majority" of British troops for the "superb job" they were doing in Iraq.
Mr Blair said allegations in the February Red Cross report had not been passed to ministers earlier because they were already being probed.
He argued there was no evidence of systematic abuse by British soldiers - saying the Daily Mirror pictures of apparent mistreatment of Iraqis by UK soldiers were "almost certainly fake".
And later, speaking in Downing Street, he said he had become "very angry" on behalf of troops because of all the allegations "swirling around".
The newspaper later issued a statement calling on Mr Blair to produce conclusive evidence that the pictures were fakes, if that's what he thought.
Two reports emerged this week from human rights groups: the Red Cross concerns about Iraqi prisoners, and an Amnesty International report about civilian casualties.
Mr Blair said that 33 cases of deaths of civilians in Iraq had been probed and there was no case to answer on 15. Decision on action over six more would be announced shortly.
He added: "Any abuse by any coalition forces is completely unacceptable.
What is not true is that allegations were made and nothing happened in respect
The Conservatives are also pressing ministers to end confusion about whether Britain's then special representative to Iraq, Sir Jeremy Greenstock, saw the Red Cross report in February.
Downing Street initially suggested he had but later said the report had been handed to Sir Jeremy's legal advisers and not read by the diplomat himself.
BBC correspondents say some ministers privately fear Britain is being tainted by the behaviour of some American troops.
Former Labour chief whip Derek Foster, who backed military action in Iraq, told BBC Five Live many backbenchers were "uneasy about the perceived closeness of the relationship with the United States".
Mr Foster said problems in Iraq were doing the party damage.
Earlier the beheading of American Nick Berg was condemned as a "barbaric act" by Downing Street.
Mr Berg was murdered apparently in revenge for the mistreatment of Iraqi prisoners in the hands of US soldiers.
"There is no justification for this kind of act in a civilised world," Mr Blair's spokesman said.
The Bush administration has vowed to catch the killers of Mr Berg, who filmed themselves cutting off his head after seizing him in Iraq.
There has been widespread revulsion in America after a video of his killing was shown on an Arabic-language website
Former Archbishop of Canterbury Lord Carey condemned the beheading as "barbaric".
He also described as "shameful" the abuse of Iraqi prisoners.
"It's cruel, it's horrible... it can so easily lead into direct violence as that decapitation has done," He told BBC Radio 4's Today programme.