Six people are missing following an ambush by suspected jihadists on a humanitarian convoy in northeast Nigeria's Borno state, an aid group said on Friday.
Action Against Hunger (ACF) said in a statement that a member of staff was among the missing after the convoy was ambushed in Kennari, a village outside Damasak Town near the border with Niger.
"One of the drivers was killed, while one Action Against Hunger staff member, two of the drivers and three health workers are missing," ACF said.
"We are deeply saddened by this tragic incident as these are colleagues dedicated to providing life-saving assistance," they added.
A statement by the United Nations' humanitarian coordinator, Edward Kallon, said he was "deeply disturbed" by the attack.
"I call on all who may have influence to do everything they can to keep them unharmed and work towards their safe return," he said.
An Islamic State-affiliated group, the Islamic State West Africa Province (ISWAP), has been active in the area, repeatedly attacking military bases.
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The three-vehicle convoy was returning to Damasak from Layi village where ACF runs a clinic for locals when the gunmen opened fire, according to a humanitarian source who spoke on condition of anonymity.
One of the vehicles veered off the road and crashed into a tree, killing the driver.
"The four were stuck in the vehicle and seized by the attackers," the humanitarian source said.
"The occupants in the other vehicles abandoned their vehicles and fled on foot to Damasak, eight kilometres (five miles) away," the source added.
The jihadist insurgency in northeast Nigeria has been raging for a decade.
Further complicating the conflict has been a split within Boko Haram. In 2016, ISWAP, a group that has sworn allegiance to IS leader Abu Bakr al-Baghdadi, broke away.
Since 2009, more than 27,000 people have been killed, some two million have fled their homes and milliions are dependent on aid.
NGOs have increasingly been targeted by jihadists.
Two female aid workers with the International Committee of the Red Cross (ICRC) were murdered by ISWAP last year and an aid worker with the UN children's agency Unicef is still being held by the group.
The trio were seized during a raid in the remote northeast town of Rann where three Unicef staff were killed along with eight soldiers.