The state governor says a bomb blast in Nigeria's northern university town of Zaria has killed 20 people.

Gov. Nasir el-Rufai of Kaduna state is urging citizens to avoid crowded public places including mosques and churches as Islamic extremists Boko Haram extend the breadth and tempo of attacks that have killed some 300 people in a week.

El-Rufai used Twitter to report Tuesday's "terrorist bomb attack." Local media said the explosion occurred at the government headquarters as civil servants were assembling for a training course.

Nigeria's police force announced increased security around mosques and churches after Boko Haram bomb and shooting attacks Sunday killed more than 60 people in a mosque and posh Muslim restaurant in central Jos city and at an evangelical Christian church in northeastern Potiskum town.