Islamist extremists threatening a bloodbath forced thousands of people out of villages in Nigeria’s northeast, where refugees said Saturday that the fighters have regrouped after a month-long military crackdown.

People who had escaped from the bush near Nigeria’s border with Cameroon and fled to Maiduguri, the capital of Borno state, said that militants from the Boko Haram terrorist network have also written letters warning government workers to resign under threat of death. Other villagers left for Cameroon.

“They warned government officials and civil servants in Bama [district] to resign or else face death in the next seven days. We are all scared this could be more deadly, so we ran for our dear lives,” said Abba Fannami, who fled to Maiduguri with six family members.

A police officer said Boko Haram fighters were ransacking homes in neighboring Gwoza district, forcing residents to hide in caves in the rocky hills.

Soldiers and police on bombing raids with jet fighters and helicopter gunships have dislodged the insurgents from camps in a game reserve. But the refugees confirmed reports that the fighters have regrouped in the mountains and rocky hills of Gwoza and Bama.

In recent days, the extremists — whose name means “Western education is sacrilege” — have killed 16 high school students and two teachers in two school attacks. The militants have also burned down at least 50 primary schools in the past year, according to Borno state’s commissioner for primary education, Tijjani Abba Ali.

In a separate operation in Kano state, west of Borno, police said they rounded up 400 migrants on Saturday and are deporting those without the necessary documents.

“This is a mop-up exercise of illegal immigrants to complement what the federal government is doing in the states currently under the emergency rule,” Kano state controller Hamisu Maishanu told reporters.

By late Saturday morning, he said, screenings had revealed about 150 people from the neighboring country of Niger without the right documents.

Kano state is not in a state of emergency. The emergency covers the northeastern states of Adamawa, Borno and Yobe, an area encompassing 60,000 square miles.

The emergency and a military and police crackdown since May 14 have failed to crush the extremists blamed for killing more than 1,600 people since 2010. Boko Haram and splinter groups want to create an Islamist state in Nigeria, Africa’s most populous nation and the continent’s biggest oil producer.

— Associated Press

Ibrahim Shuaibu in Kano city contributed to this report.