More than 80,000 people have fled their homes in Pool province surrounding Republic of Congo's capital since the government began a military operation there last year, a joint UN and government statement said.
The campaign, involving occasional aerial bombardments, aims to curb what the government says is a resurgent rebellion led by Pastor Ntumi, an enemy of President Denis Sassou Nguesso from the oil-rich country's 1997 civil war.
While it has been hard to confirm death tolls and the impact on residents, any clear evidence of escalating violence could be damaging to Sassou Nguesso's ruling party, the Congolese Party of Labour, ahead of legislative elections next month.
The United Nations is seeking around $20 million in emergency funding to provide humanitarian assistance in the province, after a recent visit found widespread signs malnutrition, the statement released late on Friday said.
Many of the displaced remain beyond the reach of aid workers, it added.
"In non-accessible zones... there is reason to fear an even more complicated situation as the number of (displaced) continues to increase and living conditions worsen more every day."