Boko Haram Islamists have published a video showing three kidnapped members of an oil exploration team, after an ambush in northeast Nigeria earlier this week that killed at least 50.
In the four-minute video, the trio identify themselves as being from the University of Maiduguri and call on the government to meet the jihadists' demands in exchange for their safe return.
The men were part of a Nigerian National Petroleum Corporation (NNPC) team on a mission to find commercial quantities of oil in the Lake Chad basin when they came under attack on Tuesday.
"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 promised 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 the convoy came under attack near Magumeri, some 50 kilometres (31 miles) by road northwest of Maiduguri.
Most of the victims were soldiers and civilian militia members providing security.
Five members of staff from the university -- two lecturers, two technologists and a driver -- were also killed, vice-chancellor Ibrahim Njodi said on Friday.
University of Maiduguri spokesman 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.
Experts said the attack -- Boko Haram's bloodiest this year -- underscored the persistent threat posed by the jihadists, despite government claims the group is a spent force.
"It's a confirmation of the boldness and reassurance that Boko Haram has managed to gain over the last six weeks," Yan St-Pierre, from the Modern Security Consulting Group in Berlin, told AFP.
"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."
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 in suicide and bomb attacks.
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 and deepened the country's worst recession in decades.
The military gave the NNPC the green light to continue exploration in November 2016, according to Nigeria's junior oil minister Emmanuel Ibe Kachikwu.
The University of Maiduguri's vice-chancellor told reporters on Friday the university had been hesitant to send staff with the NNPC team but had been assured about security.
Kidnapping has been a feature of the eight-year Boko Haram insurgency, which has killed at least 20,000 people, displaced more than 2.6 million others and left millions of others on the brink of famine.
Thousands of women and girls have been seized, while men and boys have been made to fight in the Islamist ranks.