The United Nations secretary general sacked security co-ordinator Tun Myat after a scathing report on last year's bomb attack on the UN's HQ in Baghdad.
The UN's headquarters were gutted by the blast last August
But Kofi Annan refused an offer to resign from his deputy Louise Frechette, his spokesman Fred Eckhard told reporters at the United Nations.
Mr Eckhard said the failures had been "collective", except in the case of Tun Myat, who had specific responsibility.
UN special envoy Sergio Vieira de Mello was among the 22 people killed.
More than 100 others were hurt as suicide bombers apparently drove a truck laden with explosives into the compound on 19 August.
In other developments in Iraq:
- A roadside bomb kills a US soldier near the flashpoint town of Falluja, west of Baghdad, the US military says
- The US Army admits responsibility for the deaths of two Iraqi journalists working for the Arab al-Arabiya satellite television on 18 March in Baghdad. It says it regrets what it describes as the accidental shooting
'Delusion' of security
A 30-page summary of the report - circulated internally within the UN at the beginning of March - was released to journalists on Monday.
The report suggests that UN officials failed to ask searching questions before deciding to return UN staff to Baghdad, under heavy international pressure.
The report was particularly critical of two UN officials in Baghdad, accusing them of "a dereliction of duty" and "a lethargy that is bordering on gross negligence" for
failing to shield the office windows with blast-resistant film.
The two - Jordan's Paul Aghadjanian and Pa Momodou Sinyan of Gambia - were charged with misconduct and will face disciplinary proceedings.
The report also suggests that Mr Vieira de Mello was among other senior managers who failed to appreciate a building security threat against them.
"They were living under the delusion the UN would not be attacked," Mr Eckhard said on Monday.
Mr Vieira de Mello's deputy Ramiro Lopes da Silva - who was
responsible for security on site at the Baghdad hotel where the UN staff were based - will be reassigned elsewhere with no security duties, Mr Eckhard said.