Gunmen have killed 20 people, including a traditional leader, in an attack on a village in northern Nigeria's Zamfara state, a local official has said.The attackers stormed Kaboro village and began shooting indiscriminately.Their motive was robbery and they fled after raiding houses for cash and other valuables, the official added.At least 23 people were killed in June in villages in Zamfara by gangs of robbers on motorbikes. Some of the victims' throats were slit.In Kaboro, the village chief appealed to the attackers to stop shooting but they turned their weapons on him instead, AFP news agency quoted the official, Zamfara state government spokesman Nuhu Salihu Anka, as saying.The Nigerian Tribune newspaper reports that the gang - armed with AK-47 rifles - came to Kaboro on horses and motorcycles and their attack lasted for about two hours.Militant Islamist group Boko Haram has previously carried out a number of attacks in northern Nigeria, but there has been no suggestion that that it was connected to this violence.Nigeria has one of the highest crime rates in Africa and is notorious for gun-related violence, including kidnapping and robbery.Some communities have formed vigilante groups to fight the robbers.