Secretary-General Kofi Annan (search) said Wednesday that both the United States and the United Nations (search) made mistakes when it came to Iraq, and security was being reassessed after a bomb killed at least 20 people at U.N. headquarters in Baghdad.

"The coalition has made some mistakes and maybe we have, too," Annan said, returning to U.N. headquarters after cutting his summer vacation short. "I don't want to get into finger pointing but along the way mistakes have been made by all concerned."

Annan said the international organization which provides humanitarian relief to millions of Iraqis would not be deterred by Tuesday's attack.

"We will persevere. We will continue. It is essential work," Annan said earlier in the day in Stockholm, Sweden. "We will not be intimidated."

He said a multinational force for Iraq was under discussion but it will not be a U.N. force.

"I do not see U.N. blue helmets going into Iraq at this stage," Annan said.

Annan was to meet with the Security Council (search) later Wednesday to discuss ways to better protect U.N. workers in Iraq.

On Tuesday, an explosives-packed truck detonated outside U.N. offices in Baghdad. The explosion blew a 6-foot-deep crater into the ground, ripped apart the building's facade and injured 100 people.

"We are reassessing our security arrangements in Iraq. We have been in Iraq for 12 years and we have never been attacked," Annan said, expressing shock at how a U.N. mission was targeted by an apparent suicide bombing. Unlike U.S. occupation forces, the organization had been welcomed by many Iraqis.

Annan said he was mourning those killed, including the top U.N. envoy in Iraq, Sergio Vieira de Mello (search), whom Annan called a "brilliant colleague." The United Nations will continue its humanitarian mission in Iraq so the victims' deaths "shall not be in vain."

Vieira de Mello began work June 2 and would have finished his assignment at the end of September. A 55-year-old veteran diplomat from Brazil, Vieira de Mello was trapped in the rubble and died within hours.

U.N. workers were told to stay home Wednesday following what was believed to be the first suicide bombing ever against a world body facility. The United Nations distributes humanitarian aid and is developing programs aimed at boosting Iraq's emerging free press, justice system and monitoring of human rights.

Annan appeared to indirectly criticize the United States for having underestimated the difficulties of pacifying and rebuilding Iraq following its March invasion to topple Saddam Hussein.

"We had hoped that by now, the coalition forces would have secured the environment for us to be able to carry on ... economic reconstruction and institution building," Annan said. "That has not happened."

But he also recognized the difficulty in bringing stability to postwar Iraq.

"Some mistakes may have been made, some wrong assumptions may have been made, but that does not excuse nor justify the kind of senseless violence that we are seeing in Iraq today," Annan said.

"These extremists who are targeting innocent civilians are not doing their nation or the people of Iraq any service."

Annan said he hoped those responsible for the attack would be brought to justice — although that may be a long way off. The unidentified perpetrators appear to be organized and sophisticated, Annan said.

"There are those who believe that these are limited efforts by a small group of people trying to undermine the work of the coalition. Others have described it as a guerrilla war," he said.

"Obviously, it seems to be much more organized and much deeper than one thought at the beginning."

Leaders around the world also vowed not to let it deter the world body's efforts to rebuild Iraq.

"The process of Iraq moving toward independence and steady development will not be stopped and the U.N. mission will not be suspended either," the official Xinhua news agency quoted Chinese President Hu Jintao as saying.