At least 11 people have been killed in a bomb attack on a police convoy in north-eastern Nigeria, police say."A bomber on a motorcycle rammed into the police rider [motorcycle escort]," a police spokesman said.The attack in the normally quiet town of Jalingo comes a day after at least 20 people were killed at churches elsewhere in northern Nigeria.No group has said it carried out the bombings but militant group Boko Haram has carried out many such attacks.It has carried out several suicide bombings across the mainly Muslim north of Nigeria, where it wants to establish Islamic law.The attack in Jalingo targeted local police commissioner Mamman Sule, according to a police spokesman quoted by AP news agency.Reports say the official was travelling to work in the convoy in Taraba state, on the border with Cameroon.Mr Sule was unhurt but the huge explosion ripped through a nearby market and the local finance ministry offices.At least 22 people were wounded in the blast, Red Cross officials said.On Sunday, churches were targeted in northern Nigeria's main city of Kano, as well as Maiduguri, where Boko Haram first emerged.Boko Haram, whose name means "Western education is forbidden", has targeted government institutions, churches and bars as well as mosques belonging to rival Muslim groups across northern Nigeria.It has also attacked the UN headquarters in the capital, Abuja.