Refugees say Islamic extremists threatening a bloodbath are forcing thousands of people from villages in north Nigeria where the fighters have regrouped following a monthlong military crackdown.

People who escaped through the bush to the Borno state capital of Maiduguri say militants from the Boko Haram terrorist network also have written letters warning government workers to resign their jobs or face death.

They said some villagers fled to neighboring Cameroon.

In a separate operation in Kano state, west of Borno, police say they have rounded up 400 migrants and are deporting those who do not have the necessary documents.

A state of emergency and military and police crackdown since May 14 has failed to crush the extremists blamed for the killings of more than 1,600 people since 2010.