SOKOTO, Nigeria: Attacks targeting authorities in two major cities of Nigeria's troubled north have left two suspected suicide bombers dead and killed three others, authorities said Monday.

Simultaneous bomb attacks Monday in the major northwestern city of Sokoto also killed a civilian and a police officer, said the regional police chief, Assistant Inspector General of Police Mukhtar Ibrahim. One of the bombers struck a compound containing a police station and regional police offices, Ibrahim said. Another bomber attacked a police station about two miles (four kilometers) away, he said.

An injured man at Specialist Hospital Sokoto, who spoke on condition of anonymity because of the sensitivity of the matter, said he saw a car race into the main gate of the compound. He was on a bicycle when the blast went off. He said the impact of the blast threw him off his bicycle and that he hurt his hand in the fall.

Police sealed off roads leading to both police premises soon after the blasts.

The twin explosions come as Nigeria faces an increasing threat from a radical Islamist group known as Boko Haram.

There has also been a spate of recent attacks targeting uniformed officers, some of which have been blamed on the group.

Three gunmen killed a shoe-shiner Monday morning outside an uninhabited house belonging to Nigerian Vice President Namadi Sambo in the north central city of Zaria, said Kaduna state police spokesman Abubakar Balteh. He said the house had been under renovation and that the man was near policemen who had been guarding the construction site.

Rioters had burned down that same house during postelection violence that swept across northern Nigeria after April 2011 presidential polls, Balteh said.

Sectarian violence has risen since that violence that left at least 800 people dead across Nigeria's north, according to Human Rights Watch. Fighting started after President Goodluck Jonathan, a Christian from southern Nigeria, was declared winner. Many in Nigeria's north thought a Muslim from the north should have become president.

Two past presidents and one-time rivals said in a joint statement Sunday that "the nation is gripped by a regime of fear and uncertainty... virtually all citizens have difficulties going about their normal day to day lives without great anxiety and trepidation," said the statement signed by Ibrahim Babangida and Olusegun Obasanjo. "This cannot be allowed to continue!" it said.

The statement did not mention Boko Haram by name. However, authorities have accused the group of trying to exacerbate religious tensions in Africa's most populous nation evenly divided between Muslims and Christians.

Over the last few days, attacks against security authorities, a typical Boko Haram target, have spanned a wide geographical area.

Air Commodore Sani Ahmed said motorcycle-mounted gunmen killed two air force officers in the northern city of Kano on Sunday.

The violence followed a Friday night clash between suspected Boko Haram members and security officers in the northeastern city of Damaturu that left a policeman and a soldier dead, said Yobe state police spokesman Toyin Gbadagesin. He added that security officers then razed a house believed to be harboring group members.

Witness Yau Zadawa said two motorcycle-mounted gunmen also killed a policeman outside his house late Friday after he had closed from a shift guarding a local politician in the northeastern city of Bauchi.

Meanwhile, a soldier was also shot dead Friday in the Boko Haram group's spiritual home of Maiduguri, a city about 280 miles (460 kilometers) away from Bauchi.

Security officials are frequently targeted in violence in Nigeria's arid north and have been criticized for killing suspects in their attempt to stop spiraling sectarian violence.