ABUJA, Nigeria — Gunmen on Tuesday targeted a convoy of U.S. Embassy staffers in southeast Nigeria, killing two of its local workers and two policemen, the police said.

The assailants opened fire on the convoy along a major road in Ogbaru local government area in Anambra State, one of the epicenters of separatist violence in the region, according to the police. "The hoodlums murdered two of the Police Mobile Force operatives and two staff of the Consulate, and set their bodies ablaze and their vehicles," said Tochukwu Ikenga, a police spokesman in Anambra.

A joint team of security forces was deployed to the scene but arrived only after the assailants escaped with two other police officers and one of the drivers, Ikenga added. He said no U.S. citizen was on the trip.

U.S. State Department said its personnel in Nigeria are working with the nation's security agencies to investigate Tuesday's attack. "The security of our personnel is always paramount, and we take extensive precautions when organizing trips to the field," the State Department said in a statement.

It is not immediately clear the nature of the trip embarked upon by the U.S. embassy staffers in Anambra, nor how many there were in the convoy. Ikenga said it was regrettable that "a convoy of such or any related will enter the state without recourse to the police in the area or any security agency."

The attack in Atani town, located 60 kilometers (37 miles) from the state capital, further raised concerns about the safety of residents and travelers amid the separatist violence that has become rampant in Nigeria's southeastern region in recent years.

Authorities have blamed the violence on a separatist group known as the Indigenous People of Biafra which is leading a campaign for the region to break away from the West African nation to form an independent country. The separatists have become more violent in the last few years as they continue to demand a referendum and since the trial of its leader Nnamdi Kanu on terrorism charges.

Nigerian President Muhammadu Buhari has rejected the calls for a referendum, insisting that the unity of Africa's most populous country — and the continent's largest economy — is not negotiable.

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