LAGOS/KANO: At least 25 people were killed when a bomb blast ripped through a packed government office in Zaria, northern Nigeria, on Tuesday, in a suspected suicide attack likely to be blamed on Boko Haram.

Gov. Nasir el-Rufai of Kaduna state is urging citizens to avoid crowded public places including mosques and churches as 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.

The explosion happened at about 9:00 a.m. (0800 GMT), as primary school teachers and public sector workers were queuing for identity checks, according to witnesses.

“I am sad that a terrorist bomb attack just killed 20 people in Sabon Gari Zaria,” Kaduna state governor Nasir Ahmad El-Rufai wrote on his Twitter account.

But witnesses feared the death toll could rise further, describing how the building was packed with people at the time. Zaria, a mainly Muslim city some 75 km north of the state capital Kaduna city, is near the border with Katsina and Kano states.

On Monday night, a young girl believed to be aged just 13 was killed when explosives strapped to her body detonated near a major mosque in Kano city.

That bombing followed a twin attack on a mosque and restaurant in the central city of Jos on Sunday, which left 44 people dead, and a suicide bomb attack on a church in Potiskum that killed five.

Boko Haram has increased the intensity and frequency of its attacks on civilians since President Muhammadu Buhari came to power on May 29 vowing to crush the rebels. But he been unable to stem the bloodshed from guerrilla-style attacks on “soft” targets.

With the latest blast, nearly 550 people have been killed since Buhari’s inauguration, according to AFP reporting.

Local government worker Mahmud Abbas said public sector staff and primary school teachers were undergoing screening to identify potential ghost workers on the state payroll.