Kano- At least 69 people are known to have died in a Boko Haram ambush of an oil exploration team in northeast Nigeria, as three men kidnapped by the jihadists made a video appeal. Experts said the attack - Boko Haram's bloodiest this year - underscored the persistent threat it poses, despite government claims the group is a spent force."So far the death toll stands at 69," said an aid agency worker involved in the recovery of bodies after the attack in the Magumeri area of Borno state on Tuesday.The worker, who asked not to be identified because he was not authorised to speak to the media, said 19 soldiers, 33 civilian militia and 17 civilians were killed."The last body was recovered Friday in the bush in the Geidam district of neighbouring Yobe state, which is several kilometres from the scene of the ambush," he told AFP."It shows the victim, who had gunshot wounds, died after trekking a long distance. There could be more such victims in the bush."Another source with knowledge of the rescue operation gave the death toll as "70 or more" and also said it was unclear whether all the victims had been accounted for.The attack hit Nigerian National Petroleum Corporation staff."It's a confirmation of the boldness and reassurance that Boko Haram has managed to gain over the last six weeks," said Yan St-Pierre, from the Modern Security Consulting Group."They have been attacking more and more military outposts and more military convoys. For them to go after NNPC personnel just shows they don't fear any military reprisal."Basically they have managed to gain enough resources, enough material, to plan ambushes targeted towards high value targets."Video appeal News of the rising death toll came after Boko Haram published a four-minute video in which three men identified themselves as being from the University of Maiduguri.The trio were part of a NNPC team on a mission to find commercial quantities of oil in the Lake Chad basin."I want to call on the acting president professor Yemi Osinbajo to come to our rescue to meet the demand," one of the men says in the video, which he said was shot on Friday.He attributed the attack to the Islamic State-supported Boko Haram faction headed by Abu Mus'ab Al-Barnawi, which has vowed to hit military and government targets."They have promised us that if their demands are met they will release us immediately to go back to the work we were caught doing," the man added.There was no indication of where the video was shot but Magumeri is some 50km by road northwest of Maiduguri.University of Maiduguri spokesperdson Danjuma Gambo confirmed the identities of the three kidnapped men in the video."They are our staff but one more is yet to be accounted for," he told AFP.Five members of staff from the university - two lecturers, two technologists and a driver - were killed, vice-chancellor Ibrahim Njodi said on Friday.He told reporters the university had been hesitant to send staff with the NNPC team but had been assured about security.Nigeria is searching for oil in the northeast to try to reduce its reliance on supplies from the Niger delta, where militant attacks have slashed production. Suicide bombingsKidnapping has been a feature of the Boko Haram insurgency, which has killed at least 20 000, displaced more than 2.6 million and left millions of others on the brink of famine.Thousands of women and girls have been seized, to be married off to fighters, used as sex slaves or suicide bombers, while men and boys have been made to fight in the Islamist ranks.The al-Barnawi faction differs from fighters loyal to Boko Haram's long-time leader Abubakar Shekau in that it disagrees with the indiscriminate targeting of civilians.On Friday, two suicide bombers struck a camp for displaced people in Dikwa, 90km east of Maiduguri, killing five.Civilian militia member Babakura Kolo said the men disguised themselves as traders wanting to buy grain.In Bama, 70km southeast of Maiduguri, three young female suicide bombers were killed when their explosives detonated prematurely on Thursday, he added."A fourth bomber, an 11-year-old girl, was too frightened to pull the trigger. She succeeded in removing her vest and sneaked into town. She was found and taken into custody," he said.