Geneva - The United Nations called on Nigeria on Tuesday to restore law and order in the northeast and investigate "mass killings" of civilians blamed on Boko Haram insurgents.Nigeria's military said on Monday that at least 150 people had been killed in clashes with Islamists in the north-eastern town of Baga, but the UN human rights office noted that there were "wildly differing" accounts with some reports putting the toll this year at 2 000."While the exact details remain unclear, what appears fairly certain is that mass killings and mass forced displacement have occurred," the UN human rights spokesperson Ravina Shamdasani told a news briefing in Geneva."We urge the government to act swiftly to restore law and order while ensuring that security operations are conducted in line with international law and full respect for human rights."Nigeria's military is battling to reclaim Baga from the Sunni Muslim militants, who want to establish an Islamic state in northern Nigeria.Biggest security threatBoko Haram has killed thousands in a five-year rebellion which is seen as the biggest security threat to Africa's top oil producer and is a headache for President Goodluck Jonathan ahead of what is likely to be a closely fought vote on 14 February.Opening his re-election campaign this month, he defended his record on fighting the insurgency.There is also the risk of the militants destabilising neighbouring countries. At least 143 Boko Haram fighters were killed in an attack on a military camp in Cameroon on Monday, the government in Yaounde said.At least 11 320 Nigerian refugees have fled the violence by crossing into neighbouring Chad since the start of the year, the UN refugee agency said."Some of them are stranded on an island on Lake Chad, called Kangala Island, there are about 2 000 of them there," UNHCR spokesperson William Spindler said, adding that the agency was transferring them to the Chad mainland.The UN aid agencies have not been able to verify the number of people displaced within Nigeria because of a lack of independent access to the areas affected, but stand ready to provide humanitarian assistance, OCHA spokesperson Jens Laerke said.