Story highlights Boko Haram raid kills 66 in northeastern town of Damboa in Nigeria

Town resident said they were left defenseless after fending off previous attack

Nigerian military: "We are not conceding any portion of this country to any terrorist group"

A days-long raid on a northeast Nigerian town by Boko Haram left dozens dead and displaced more than 15,000 residents, Nigeria's National Emergency Management Agency reports.

Scores of fighters armed with rocket-propelled grenades, explosives and guns descended on the town of Damboa early Thursday evening, residents and officials said.

By the time the raid ended on Sunday, 66 residents had been killed and more than 15,000 had fled, NEMA spokesman Abdulkadir Ibrahim said.

"We lost 46 people on Thursday through Friday in Damboa and the gunmen pursued people who fled to nearby Fori and Kwairi villages where they killed 20 more," Sheriff Muhammad, a spokesman for the Damboa local authority, said.

The Islamic militant group's trademark black flag was hoisted afterward, signifying that the town was under their control.

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"Boko Haram have taken over Damboa where they erected their flags after the attack that forced all residents to flee," said Sheriff Mohammed.

But the Nigerian military tried to assure residents that it would retake the town.

"We are not conceding any portion of this country to any terrorist group," said Maj. Gen. Chris Olukolade, a Defense Ministry spokesman. "We are going to reverse every form of insecurity in that area very soon."

Boko Haram launched an offensive against Damboa's military base and police station July 4 that left six soldiers and five policeman dead, according to Nigeria's defense ministry. But 50 militants were also killed that day, leading residents such as Ahmed Buba to think they had returned to exact revenge.

"We were defenseless because all the security personnel, including soldiers and policemen, have withdrawn," Buba said.

Boko Haram -- whose name translates to "Western education is forbidden" in the Hausa language -- caused international outrage when it abducted 276 girls on April 14 from a Nigerian boarding school.