Thousands of men, women and children made homeless by the Boko Haram insurgency risk disease because of lack of shelter in northeast Nigeria, aid workers said on Tuesday.
The Norwegian Refugee Council said more than 4,000 people were having to sleep in the open in the town of Dikwa, where they have fled military operations against the terrorists.
Nigeria, which maintains the terrorists are virtually defeated, is encouraging internally displaced people (IDPs) to return to their homes, as troops wind up operations.
But humanitarian organisations say towns outside the Borno state capital, Maiduguri, do not yet have the minimum standards of basic services to cope with an influx of so many people.
The Nigeria director of the Norwegian Refugee Council, Cheick Ba, said they were "extremely concerned" by the situation in Dikwa, which is 92 kilometres (57 miles) east of Maiduguri.
"Children are sleeping outside with nothing over their heads. With the rains now hitting the area, they risk becoming sick with malaria, diarrhoea or typhoid," he said in a statement.
There were 1.7 million IDPs in Borno state and neighbouring Yobe and Adamawa at the end of May, the United Nations said last week.
But "large-scale displacements" were happening every week as a result of increased fighting this year. In May, 21,207 people arrived in five towns in Borno state, including Dikwa.
Resources were stretched and $41.7 million was needed to provide life-saving assistance to some 115,000 IDPs who are expected to move in the coming months, it added.
The NRC said aid agencies were "overwhelmed" and hundreds of IDPs had arrived in Dikwa since April because of fighting between the military and Boko Haram in surrounding areas.
More than 600 people awaiting military screening were staying in an unused petrol station which has no roof; 4,000 others who have been screened were at a reception centre.
But the centre was "full to the brink", forcing families to sleep in the open. Women crammed into single rooms with men outside was a common sight, the aid agency added.
The need for emergency shelter was immediate, as IDPs had also taken over four local schools while the military was using another as a base.
There was no immediate response from Nigeria's defence spokesman when contacted by AFP.