MAIDUGURI, Nigeria – Nigerian police and immigration services are on high alert after officials said Monday they received text messages threatening new religious violence by a radical Islamic sect.

Officers set up checkpoints in armored tanks around Maiduguri, the capital of Borno state where rioting by the Islamic sect Boko Haram and an ensuing police crackdown left 700 people dead. Immigration agents in the state that borders Chad, Niger and Cameroon also are watching for Boko Haram members who may cross the border to spark new violence in the area, said Adamu Isa Azare, an assistant superintendent of police in Borno.

Azare said the increased security presence comes after police received text messages that promised the group would rise again and attack around Maiduguri. Violence between Christians and Muslims in central Nigeria has left more than 500 people dead since the start of the year, sparking recent calls from an al-Qaida-affiliated Web site for a Muslim uprising against Christians.

"We are not (sure) if it is Boko Haram per se, because some people we are yet to identify were just sending text messages to people that there was going to be attacks," Azare said. "For this reason, we stepped up our security apparatus as proactive measures to ensure we were not caught unaware. We want to ensure the people are well-policed. We don't want to take anything for granted."

Azare also asked the public to call authorities if they saw large groups of unfamiliar people moving into the region.

Boko Haram — which means "Western education is sacrilege" in the local Hausa language — has campaigned for the implementation of strict Shariah law. Its members rioted and attacked police stations and private homes in late July, sparking the police crackdown. Authorities have been accused of killing Boko Haram leader Mohammed Yusuf while he was in custody. Police officials said he was killed while trying to escape, but army officials said he was alive when he was arrested.

The group largely went underground after Yusuf's death. In early March, police arrested 17 officers suspected of taking part in filmed executions that later aired on international news channel Al-Jazeera.