At least 30 police officers in Nigeria have been killed in an ambush by a local militia in the Nasarawa state, officials have said.They were on their way to arrest the leader of the outlawed Ombatse "cult" when gunmen opened fire, a state spokesman told the BBC.Sani Musa Mairiga said they were forcing local villagers to swear an oath of allegiance to the group.The state police chief said that 17 officers were still missing. One source at the hospital where the bodies were taken told the BBC dozens more policemen were killed in the attack.Nigeria's President Goodluck Jonathan has aborted his visit to South Africa and cancelled a state visit to Namibia to oversee the response to the latest violence.Nasarawa police Chief Abayomi Akeremale said about 60 police officers came under attack on Tuesday."We decided to send our men to the area to arrest members of Ombatse, including their priest," he told the AFP news agency. "[They] have been going to churches and mosques initiating people into their cult by forcefully administering an allegiance oath to unwilling people."The police were attacked near the shrine to the traditional deity of the Eggon people, in the village of Alakyo, near the state capital, Lafia.The ambush came after 55 people were reportedly killed in the north-east of Nigeria in coordinated attacks by the Boko Haram Islamist militant group.