The bombing on Tuesday took place in Goldavi, near the border with Nigeria.
About 18,000 refugees are staying in the region.
"We are appalled by these senseless attacks on people who have been uprooted from their homes, fleeing violence, being chased by armed gangs ravaging the region - only to lose security again as they have just found refuge elsewhere," said Olivier Guillaume Beer, UNHCR. Cameroon's representative in Cameroon.
According to the UN agency, violence has escalated in northern Cameroon recently. The Islamist extremist movement Boko Haram and other armed groups regularly carry out deadly acts and kidnappings in the region.