Monday, UN Secretary-General Ban Ki-moon condemned the slaying of three North Korean doctors knifed to death in northeastern Nigeria.
The doctors, as well as other health workers slain earlier, "were working selflessly to provide basic life-saving interventions to the most vulnerable people," Ban said in a statement released by his spokesman Martin Nesirky.
"These acts of outrageous violence toward health workers around the world are unacceptable," said Ban, who encouraged "all countries to protect health workers so that they can continue to better the health of populations and save lives."
The North Koreans were in Nigeria as part of a cooperation agreement signed some five years ago, said Abdullahi Bego, spokesman for the governor of Yobe state, where the three were killed.
Men armed with knives slit the doctors' throats at their homes in a pre-dawn attack, Nigerian police said Sunday, in the latest such killings in recent months.
Much of northern Nigeria is beset by violence from the militant group Boko Haram, which has said it is fighting to create an Islamic state in the mostly Muslim region. The southern half of Nigeria is mostly Christian.