Secretary-General Kofi Annan has refused to accept the resignation of his deputy, Louise Frechette of Canada, over the 19 August bombing of the UN's Baghdad office that killed 22 people, his spokesman said on Monday.
But Annan asked for and has received the resignation of UN security chief Tun Myat after an independent panel found serious shortcomings in the handling of security surrounding the blast.
Meanwhile the number two official in the UN's Baghdad operations at the time of the blast, Ramiro Lopes da Silva, will be reassigned to a post that has no responsibility for security, spokesman Fred Eckhard said.
A 30-page summary of the panel's report released to reporters outlined glaring errors in the handling of the UN's decision to return to Baghdad in May after the US-led war toppled Saddam Hussein.
Frechette tendered her resignation after the report was circulated internally earlier this month but Annan refused it, her spokesman said.
"She was acknowledging the criticism," Eckhard said. "The secretary general said: 'It wasn't your individual responsibility, it was the collective responsibility.'"
In a separate development, Annan has said that members of the Security Council are working on creating a multinational force to help bolster security for an elected government in Iraq.
"My impression is that they are working for a multinational force, under a unified command, which would cooperate with the sovereign government of Iraq and contribute to the security of the country," Annan told Italy's Corriere della Sera newspaper on Monday.
The UN has previously demurred on any deployment of its blue-helmet troops to Iraq, where US-led forces, without UN backing and in the face of opposition from Security Council members, last year led an invasion to overthrow Saddam Hussein.
Despite differences over the justifications for going to war, the US administration has in recent months urged the UN to take a more active role in Iraq, where US troops still face daily insurgent attacks a year after Saddam's fall.