Boko Haram Islamist militants have attacked the north-east Nigerian city of Gombe, residents say. Fighters overran a checkpoint on the edge of the city and explosions and gunfire could be heard, witnesses added. Nigerian troops backed by a fighter jet were said to be trying to drive the militants back. Nigeria has postponed elections due to be held on Saturday amid concerns over the insurgency in the north-east. Reports said militants first attacked the town of Dadin Kowa, about 40km (25 miles) from Gombe. Ground troops with air support then battled to keep the insurgents from entering the city. One resident quoted by AFP news agency said Boko Haram gunmen were at a roundabout in the city centre, "firing indiscriminately and throwing pamphlets calling on people not to participate in the elections" Its insurgency has become a regional crisis with the four affected countries - Nigeria, Niger, Chad and Cameroon -agreeing to form a regional military force to try to cont