ABUJA, Oct 23 (Reuters) - Gunmen attacked a jail in Nigeria"s Oyo State late on Friday and freed over 800 inmates by force, the third such major attack this year, prison service said in a statement on Saturday. The prison service said the attackers were heavily armed and after an exchange of gunfire with prison officers, gained entry to the prison yard by blasting the walls with dynamite. Some 575 inmates, who were all awaiting trial, were missing while 262 escapees had since been recaptured, it said, adding that the jail housed only 64 convicts, who did not escape. "While all the awaiting trial detainees were forced out of custody, the cells housing the convicts and the female inmates were not vandalised," the prison service said. Nigeria is struggling with security problems across its vast territory, including a spate of abductions of students for ransom by criminal gangs in the northwest and an Islamist insurgency in the northeast. The jail attack in Oyo follows similar attacks in Imo state in April where more than 1,800 inmates were freed, while another 266 prisoners were forcibly released in Kogi state last month. read more