Thousands of people are fleeing a northern Nigerian city, and the university there has closed indefinitely following threats from a radical Islamist sect.

The University of Maiduguri in volatile Borno State shut down Monday, and officials say it will remain closed until the security situation improves. Officials say the university received a letter from the Boko Haram sect threatening to attack the campus.

Boko Haram is blamed for a series of deadly bomb attacks and ambushes that have caused an exodus from Maiduguri in recent days. Residents say thousands of people are piling into cars and trucks to leave the city.

Boko Haram wants to impose strict Islamic law across the north, and it rejects Nigeria's constitution and federal government.

There are fears the Boko Haram threat is spreading south to Lagos. A text message claiming to come from the sect warned people not to take government buses because they are being targeted for attack.

The News Agency of Nigeria reports that passengers have been asked to remain vigilant and cooperate with spot checks of their bags.

President Goodluck Jonathan has offered to hold formal talks with the sect to end the violence. But Boko Haram leaders say they cannot meet with security forces who are trying to destroy them.