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.
One unnamed resident of the town described what had happened for the BBC:
"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.
Who are Boko Haram?
- Founded in 2002, it initially focused on opposing Western education. Boko Haram means "Western education is forbidden" in the Hausa language
- Began fighting in 2009 to create an Islamic state in a campaign which has claimed thousands of lives
- Attacks mostly in the north-east but has also targeted police and UN headquarters in the capital, Abuja
- Was declared a terrorist group by the US in 2013
In April 2013, at least 37 people were killed and 2,275 homes destroyed in Baga by troops hunting Boko Haram fighters who had attacked a patrol, Human Rights Watch reported.
The New York-based organisation accused soldiers of engaging "more in destruction than in protection".
Before Saturday's attack on the town, the area was already a stronghold of Boko Haram, which has been waging a bloody insurgency to create an Islamic state since 2009.
More than 2,000 people were killed in militant violence last year in the north-east.
Days before the attack on Baga, suspected Boko Haram militants kidnapped about 40 boys and young men in a raid on the remote Borno village of Malari.