GENEVA (Reuters) – Nigerian refugees who fled Islamist militants are returning from Cameroon into a country that is still not equipped to support them, and they risk creating a new humanitarian crisis, the head of the U.N. refugee agency, Filippo Grandi, said on Wednesday.

The UNHCR issued a similar warning last month when about 12,000 refugees returned to the border town of Banki in Borno state, which was already housing 45,000 displaced Nigerians.

Another 889 refugees, mostly children, arrived in Banki on June 17 from Minawao camp in Cameroon, Grandi said.

“The new arrivals – and we hear reports of more refugees seeking to return – put a strain on the few existing services, he said in a statement. “A new emergency, just as the rainy season is starting, has to be avoided at all costs.”

“It is my firm view that returns are not sustainable at this time.”

Banki, once a thriving town, was razed to the ground by the time the Nigerian army retook it from Boko Haram insurgents in September 2015.

Grandi said the severely overcrowded town could not provide adequate shelter or aid and its water supply and sanitation were “wholly inadequate”, creating the risk of disease.

Although Boko Haram attacks have been fewer in recent months, more people are on the move and there are 1.9 million Nigerians displaced across the northeast, the World Food Program (WFP) said in a report last week.

“Insecurity persists in parts of Northeast Nigeria, disrupting food supplies, seriously hindering access to basic services, and limiting agricultural activities, worsening an already dire food security situation,” it said.

More than 5 million people in Borno, Adamawa and Yobe states in northeastern Nigeria have no secure food supply, WFP said.

(Reporting by Tom Miles; Editing by Louise Ireland)