U.N. Secretary-General Kofi Annan has promised to appoint experts to assess blame for security lapses that left U.N. offices in Iraq vulnerable to attack. Mr. Annan named his top deputy to oversee a security reorganization.

In a letter to staff Friday, the secretary-general says he is reviewing what he called "serious weaknesses" in the U.N. security system.

He says he is gravely concerned at the findings of the independent panel he appointed to investigate the attacks on U.N. headquarters in Baghdad.

The panel, headed by former Finnish President Martti Ahtisaari, recommended a top-to-bottom overhaul of the U.N. security system. It described the world body's security as "sloppy", and suggested those in charge should be held responsible for security failures that allowed terrorists to act.

In his letter to employees, Mr. Annan promised to take immediate action to implement recommendations of the Ahtisaari committee. He will appoint a team next week to look into the issue of accountability for security failures.

In the meantime, however, he has named the U.N.'s most senior official responsible for security, Deputy Secretary-General Louise Frechette, to oversee the reorganization of the system.

U.N. spokeswoman Marie Okabe said the secretary-general views the issues of accountability and reorganization as unrelated.

"The secretary-general will be appointing an independent panel looking into the accountability issue. That is the first thing he points out," she said. "Obviously, this expert panel will then be looking into, will be charged with the accountability issue. That's a separate issue than, right now, the current security system is under the responsibility of the deputy secretary-general, so it is natural that this evaluation is being done under her leadership."

Most international U.N. staff were pulled out of Iraq after the August 19th bombing that killed 22 people, including the secretary-general's special envoy Sergio Vieira de Mello. Further reductions were made after another bomb attack in September, and again after the Ahtisaari report described the entire U.N. security system as dysfunctional.

Secretary-General Annan Thursday ordered the last 12 international U.N. staff to leave Baghdad, at least temporarily. The only foreign U.N. employees remaining in Iraq are in the northern part of the country, most of them working on the transfer of the Oil-for-Food Program, which will be handed over to the U.S.-led coalition November 21.