The militant group Boko Haram has seized a town and key multinational military base in north-eastern Nigeria, officials and eyewitnesses say.
A senator in Borno state said troops had abandoned the base in the town of Baga after it was attacked on Saturday.
Residents of Baga, who fled by boat to neighbouring Chad, said many people had been killed and the town set ablaze.
Baga, scene of a Nigerian army massacre in 2013, was the last town in the Borno North area under government control.
It hosted the base of the Multi-National Joint Task Force (MNJTF), made up of troops from Nigeria, Chad and Niger.
Set up in 1998 to fight trans-border crime in the Lake Chad region, the force more recently took on Boko Haram.
Boko Haram attacks towns and villages on an almost daily basis, abducting people including young boys and girls, BBC Africa analyst Mary Harper reports.
The military, which includes Western advisers and surveillance, seems incapable of dealing with the problem, she adds.
Residents who fled to Chad said they had woken to heavy gunfire as militants stormed Baga early on Saturday, attacking from all directions.
They said they had decided to flee when they saw the multi-national troops running away.
Unnamed Baga resident
“Yesterday at around 05:00 [04:00 GMT] we were woken up by heavy gunshots, and we couldn’t identify where the shots were coming from.
“They came through the north, the west and from the southern part of the town because the eastern part is only water. So, when we [went] towards the western part, we saw heavily armed Boko Haram men coming towards us.
“The soldiers were trying to repel the attack but that wasn’t going to happen because a lot of the soldiers were without their guns and some were running into the town. When you see soldiers running away into the town – what are you to do, other than to just run away as well?”
Maina Maaji Lawan, senator for Borno North, told BBC World Service civilians had run “helter skelter” – “some into the forest, some into the desert”.
Communications with the town were cut off and exact information about casualty numbers could not be confirmed, he said.
“We are very dispirited,” the senator added.
Confirming that the military had abandoned the base, he said people’s frustration knew “no bounds” over the apparent fact that the military had not fought back.
“There is definitely something wrong that makes our military abandon their posts each time there is an attack from Boko Haram,” the senator said.