The UN Refugee Agency (UNHCR) expects up to 40,000 refugees fleeing violence in areas of Cameroon to head to neighbouring Nigeria.
"Our fear, however, is that 40,000 might actually be a conservative figure in a situation where the conflict might continue," the UNHCR said in a statement issued on Tuesday.
Some 2,000 Cameroonians fleeing violence in the English-speaking parts of the country have already been registered in south-eastern Nigeria; a further 3,000 are waiting to register, the agency said.
Protests in the English-speaking parts of Cameroon demanding independence from the French-speaking remainder of the country at the beginning of October had renewed violence in the region.
Amnesty International said that 20 people were shot dead by the security forces during the clashes.
Once a German colony, Cameroon was divided up between the French and the British after World War I, but later reunited in 1961 after both parts gained their independence from the Europeans.