More than 200 people are now feared to have been killed in a series of bomb attacks and shooting sprees in northern Nigeria. The attacks are believed to be the work of the Islamist group Boko Haram. According to Human Rights Watch, the group has killed nearly 1,000 people over the past three years. Andrew Harding is a reporter with our partner the BBC. He visited the northern cities of Jos and Kano, which have both suffered heavy losses, and sent this report.