Hundreds of civilians, including many children, have been kidnapped by Boko Haram extremists who are using them as human shields in Nigeria, officials have said.
Mike Omeri, the Nigerian spokesman for the fight against Boko Haram, said several hundred people were abducted by the Islamic militants as they retreated earlier this month from Damasak in north-eastern Nigeria.
He could not specify how many were kidnapped, but local reports say as many as 500 people were taken.
Mr Omeri said the Islamic rebels went to Damasak's primary schools and rounded up students and teachers before retreating.
Troops from Chad and Niger recaptured Damasak, near the border with Niger, from Boko Haram on March 16.
The mass kidnapping happened as the extremists were fleeing the advancing troops, with information about the abductions only confirmed today.
The soldiers who recaptured Damasak found the town largely deserted.
Damasak had been held for months by Boko Haram, who used the trading town as an administrative centre.
The troops from Chad and Niger who now hold Damasak have discovered evidence of a mass grave, Chad's ambassador to the UN Mahamat Zene Cherif confirmed.
International assistance is needed for the thousands of Nigerian refugees who have fled the violence, said the head of the UN refugee agency.
Some 74,000 Nigerians have fled to neighbouring Cameroon, according to the agency.
Around 100,000 more have flooded into Chad and Niger. Troops from the three countries are now helping Nigeria to combat the militants and win back Nigerian towns.
The refugee agency will funnel more resources to Cameroon, said UN High Commissioner for Refugees Antonio Guterres during a visit to Maroua, the capital of Cameroon's Far North region. He stressed that additional assistance is needed.
"Cameroon is today not only a very important protection space for refugees, but it is in the first line of defence of the international community," he said.
The UN agency says the Nigerian crisis is one of the most underfunded in the world. In February, the agency asked for 71 million dollars (£47 million) to assist displaced people in Nigeria and neighbouring countries.
Already, that figure appears to be too low, it said this week. Thus far, it has received only 6.8 million dollars (£4.5 million) in donations, he said.