Kano - At least 25 people were killed on Tuesday when a bomb blast ripped through a government office in northern Nigeria in a suspected suicide attack likely to be blamed on the Boko Haram Islamist group.The explosion occurred in the town of Zaria at about 09:00, as primary-school teachers and public-sector workers were queuing for identity checks, according to witnesses."An IED [improvised explosive device] most likely carried by a suicide bomber" was used in the attack, which killed 25 people, including a two-year-old child, Kaduna state governor Nasir El-Rufai said.El-Rufai added in the statement posted on his Facebook account that 32 others were injured and were being treated in hospital.Zaria, a mainly Muslim city some 75km north of the state capital Kaduna city, is near the border with Katsina and Kano states.Twin attackAlso on Tuesday, witnesses said a female suicide bomber blew herself up near a military checkpoint in restive northeastern Borno state, killing four people.The attack happened at Sabon Gari village, some 110km from Maiduguri, the state capital.A senior military officer, who did not want to be identified, also confirmed the attack.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.Huge explosionBoko 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 so-called "soft" targets.With the latest blast, nearly 550 people have been killed since Buhari's inauguration, according to AFP reporting.Zaria local government worker Mahmud Abbas said public sector staff and primary school teachers had been undergoing screening to identify potential ghost workers on the state payroll.Another staff member who was waiting outside, Mohammed Abubakar, said: "We were taking turns going into the hall in batches for the screening and also to get our pay cheques."The first batch had gone in. There were almost 100 people there, including the staff, the screening committee and accountants from the banks."There was a huge explosion inside the hall followed by billows of smoke and dust."Nigeria's federal police chief, Solomon Arase, has ordered state commanders to beef up vigilance at all places of workship."Adequate security has also been ordered for venues where Muslims break [the Ramadan] fast in the evenings," he said in a statement late on Monday.Previous attacksZaria has not been known as a stronghold of Boko Haram, which has been fighting for six years to create a hardline Islamic state in northeast Nigeria, but it has been attacked before.In March last year, a prominent Salafist cleric who had criticised the group was shot dead with his wife and son as he drove home from teaching a theology class.An Islamic boarding school that he ran was previously hit by a bomb blast and gunfire in September 2012.Zaria was also where a French engineer managed to flee his captors in November 2013 after being held for nine months by the Boko Haram offshoot Ansaru.In June 2012, bomb blasts ripped through two churches in Zaria, killing several worshippers and injuring many others. Boko Haram claimed the attacks.