The United Nations High Commissioner for Refugees (UNHCR) is now preparing a "very conservative" contingency plan for as many as 40,000 people fleeing Cameroon, Antonio Jose Canhandula, the agency's representative to Nigeria, told Reuters in an interview in Abuja.
A cycle of state repression fuelling separatism has raised concerns the majority French-speaking Cameroon may face a prolonged period of violence, after soldiers shot dead at least eight people in the country's two English-speaking regions on Oct 1.
"Our fear is that the 40,000 might actually be an understatement in a situation where the conflict might continue," said Canhandula.
Cameroon's government "has sent security forces into southern Cameroon," he said, adding that the country has now closed its borders with Nigeria.
Demonstrations in Cameroon's English-speaking regions began nearly a year ago when Anglophone lawyers and teachers protested against having to work in French, saying it showed the wider marginalisation of the English-speaking minority.
One of Cameroon's English-speaking regions in the country's west borders Nigeria's southeastern state of Cross Rivers. There, the UNHCR has been registering refugees, some of whom say they are fleeing violence, according to Canhandula.
But Nigeria and Cameroon are already grappling with one of the world's worst humanitarian crises, in the Lake Chad region, with over 2 million people displaced after more than eight years of conflict with Islamist insurgency Boko Haram.
"Can you imagine having another refugee situation in a country where we are hardly coping with IDPs (internally displaced persons)?" Canhandula said.
"Every time you have a refugee situation you have it for several years... Cameroon really has to take the issues that create the feeling of exclusion very seriously," he said.
Cameroon's linguistic divide is a legacy of World War One, when the League of Nations divided the former German colony of Kamerun between allied French and British victors.