U.N. Secretary-General Kofi Annan (searchsaid Wednesday the world body would not pull out of Iraq despite an attack on its headquarters in Baghdad (search) that killed its top envoy and 19 other people.

"We will persevere. We will continue. It is essential work," Annan said at a news conference in Stockholm (search), where he stopped briefly before heading to U.N. headquarters in New York. "We will not be intimidated."

Annan said he was to meet with the Security Council later in the day to discuss ways to better protect U.N. workers in Iraq following the attack Tuesday, when a cement truck packed with explosives 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 homicide 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, whom Annan called a "brilliant colleague." The United Nations will continue its humanitarian mission in Iraq so that 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 thought to be the first homicide 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.