MAIDUGURI, Nigeria, Sept 2 (Reuters) - At least seven worshippers were killed in an attack on a mosque by a gang of armed men in Nigeria's northwest Kaduna state, police said on Saturday. The attacks have confounded Nigeria's security forces that are overstretched combating a 14-year Islamist insurgency in the northeast, violent farmer-herder and sectarian clashes in the central region, and rising attacks by a separatist group in the southeast. See here for a complete list of exchanges and delays