Secretary-General Ban Ki-moon said the United Nations will not be deterred by the "despicable and brutal" terrorist attack against a U.N. guest house in Kabul and will do all it can to help with next month's presidential runoff election.

The U.N. chief told a news conference that Afghan President Hamid Karzai has asked the Interior minister "to provide strengthened security for U.N. staff."

The United Nations is also reviewing security and will take measures to strengthen the safety of its premises, "not only in Kabul but also in other areas where we have seen the situation is very dangerous," he said. The U.N. will also be seeking support from the NATO-led forces in Afghanistan, he said.

The attack killed 11 people including five U.N. workers. At least 25 U.N. staff were staying at the guest house, most of them advisers for the Nov. 7 balloting.

The attack by Taliban militants wearing suicide vests and police uniforms in the heart of the Afghan capital sent people jumping out of windows or hopping from roof to roof to escape a fire that engulfed part of the three-story building.

It was the biggest in a series of attacks intended to undermine next month's presidential runoff election between Karzai and former foreign minister Abdullah Abdullah.

"We will continue our work, particularly on helping (the) Afghan government and people carrying on this second presidential election ... while ensuring and strengthening the safety and security of our premises," Ban told reporters.

"It is quite unfortunate fact of life that we cannot ensure 100 percent of security because of these suicidal terrorist attacks," the secretary-general said. "We must take all necessary precautionary measures in terms of our security. That's what I can tell you at this time. But we will never be deterred by these terrorist attacks."