United Nations offices and personnel have often been attacked by terrorists — leading Secretary-General Ban Ki-moon to caution Friday that the world body is increasingly perceived as a “soft,” or easy, target.

Earlier this year Mr. Ban urged U.N. members to ratify a protocol that would extend legal protection for U.N. staff to all operations involving humanitarian, political, or development assistance.

The U.N. says that in 2010 at least 28 members of its civilian staff were detained or arrested in cases considered job-related, while another 12, both civilians and police, were abducted. One civilian staff member was held captive in Sudan's Darfur region for nearly three months.

Friday's attack on the U.N. building in Abuja, Nigeria was only the latest of its kind. Following are other attacks on United Nations employees and offices over the past decade:

October 10, 2001: The United Nations stated the Taliban was physically attacking U.N. workers and destroying their equipment in Afghanistan. A spokesman said people working on behalf of the U.N. had been beaten by Taliban authorities in the cities of Kabul, Jalalabad, and Kandahar.

August 19, 2003: A truck bomb attack on the U.N. headquarters in Baghdad killed 22 people, including the head of the U.N. mission to Iraq, Sergio Vieira de Mello.

December 11, 2007: Twin car bombs destroyed the offices of the U.N. Development Program in Algiers and damaged the nearby offices of the High Commissioner for Refugees. Seventeen U.N. staff members were among the dozens of people killed.

October 28, 2009: At least six U.N. staffers were killed in an attack on an international guest house in Kabul.

April 1, 2011: Three U.N. staff members and four international security guards were killed in an attack in the northern Afghan town of Mazar-i-Sharif.

May 27 and July 26, 2011: Attacks on U.N. peacekeepers' convoys in Lebanon left 13 people injured.