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In December, more than 30 attackers stormed a house in the northern Nigeria state of Kaduna, killing two and kidnapping a French engineer working on a renewable-energy project.

Chinese construction workers also have been killed by gunmen around Maiduguri, the northeastern city in Nigeria where Boko Haram began.

In the most recent attack, assailants attacked North Korean doctors working for a hospital in Yobe state, stabbing two to death and beheading a third. No group claimed responsibility.

Foreign embassies in Nigeria have issued travel warnings regarding northern Nigeria for months. Worries about abductions have increased in recent weeks with the French military intervention in Mali, as its troops and Malian soldiers try to root out Islamic fighters who took over that nation’s north in the months following a military coup. Last week, the U.S. Embassy in Nigeria’s capital, Abuja, put out a warning following the killings of polio workers in the northern city of Kano and the killing of the North Korean doctors.

“The security situation in some parts of Nigeria remains fluid and unpredictable,” the embassy said.

Jon Gambrell reported from Johannesburg. Associated Press writers Cassandra Vinograd in London, Victor Simpson in Rome and Demetris Nellas in Athens contributed to this article.