POTISKUM, Nigeria (AP) -- North Korean doctors hacked to death by machete-wielding attackers. Women vaccinating children against polio gunned down in the street. A top Islamic cleric, whose predecessors once served as ultimate rulers in the region, nearly killed in an ambush.
These recent attacks in northern Nigeria show the changing tactics of Islamic extremists here and continuing dangers facing Africa's most populous nation, despite a buildup of soldiers and police officers, door-to-door searches by security forces and mass arrests. As the killings continue, analysts believe the fighters, likely part of the amorphous Islamic sect known as Boko Haram, slip easily in and out of Nigeria to launch attacks -- putting other West African nations at risk.
"It means Nigeria's problem will become another country's problem, such as Mali, Cameroon or Niger, or smaller countries like Guinea, Burkina Faso and Senegal," wrote analyst Jacob Zenn in a January publication by the Combating Terrorism Center at the U.S. Military Academy at West Point. "Like northern Nigeria, these countries have majority Muslim populations, artificial borders, ethnic conflicts, insufficient educational and career opportunities for youths and fragile democratic institutions."
On Sunday, officials found the corpses of three North Korean doctors in Potiskum, a town in Nigeria's Yobe state, about 500 kilometers (300 miles) northeast of the nation's central capital, Abuja. Two had their throats slit, while one had been beheaded by attackers, according to an Associated Press journalist who saw the corpses at a local hospital.
Those killings came quickly after gunmen shot dead at least nine female polio vaccinators Friday in Kano, the most populous city of Nigeria's predominantly Muslim north. A previous attack on a polio clinic in October in the city killed two police officers on guard there and highlighted the continuing suspicion some in the north have regarding the vaccines. A 2003 polio outbreak in Nigeria's north that spread across the world started from Islamic leaders claiming the vaccine would sterilize young Muslim girls -- rumors that persist today in a nation that is one of three in the world where the virus remains endemic.
Despite a promise by Nigerian President Goodluck Jonathan that the government would protect health workers after Friday's shooting, attackers killed the North Koreans and apparently slipped away undetected Saturday night. That led to questions Monday among residents of Potiskum who wondered how safe they are, despite a dusk-till-dawn curfew in the town and soldiers on manning checkpoints there.
"It is really unfortunate this is occurring in an area with a full military and police operation," resident Abdullahi Usman said.
U.N. Secretary-General Ban Ki-moon strongly condemned the killings of the doctors and health workers who "were working selflessly to provide basic life-saving interventions to the most vulnerable people," U.N. spokesman Martin Nesirky said.
The U.N. chief said these acts of "outrageous violence" are unacceptable and could have devastating effects in the fight to improve the health of people everywhere, he said.
"The secretary-general encourages all countries to protect health workers so that they can continue to better the health of populations and save lives," Nesirky said.
Yobe state spokesman Abdullahi Bego offered condolences for the dead, who were part of a technical exchange program between the state and North Korea. There are a dozen other North Korean doctors in the state, as well as engineers, according to officials.
"The Yobe state government will offer every possible support to the security agencies to track and prosecute the perpetrators of this criminal and condemnable act," Bego said in a statement. "The state government will also continue to partner the security agencies to ensure the safety of people's life and property."
There was no comment Monday from North Korea's government. Pyongyang's official Korean Central News Agency also had yet to report on the three doctors' deaths.
While no group has claimed responsibility for the attacks on the health workers, suspicion immediately fell on Boko Haram, whose name means "Western education is sacrilege" in the Hausa language of northern Nigeria. The group has waged an increasingly bloody guerrilla campaign of shootings and bombings for the last year and a half. In 2012 alone, the group was blamed for killing at least 792 people, according to an AP count.
Boko Haram, which once routinely claimed its attacks, has gone largely silent, starting speculation that the sect has split into smaller groups operating independently. That makes it increasingly difficult to know who is launching assaults in the north, like the January attack on the Emir of Kano Ado Bayero. At least three people died in the shooting and Bayero, who suffered injuries in the attack, has since left Nigeria for London with family members.