Armed groups have killed more than 100 soldiers and seized a huge stock of weapons in clashes in northeast Nigeria since December 26, a report by a group of aid agencies says.
An army spokesman denied the number of soldiers killed was that high.
Attacks by militants have intensified over the past few weeks and forced thousands of people to flee to safer areas in Nigeria and over the border to neighbouring Chad, Friday's report said.
The attacks have mostly been carried out by an Islamic State-allied faction of the Boko Haram group.
The surge has occurred in the run-up to an election on February 16 in which President Muhammadu Buhari is seeking a second term, and his critics have seized upon security as an issue.
The report by the Global Protection Cluster in Chad, a group of aid agencies led by the United Nations High Commissioner for Refugees, said raids against the Nigerian army have killed more than 100 Nigerian soldiers.
"According to the information available, the armed groups captured a huge stock of weapons," it said.
Nigerian Army spokesman Onyema Nwachukwu said the casualty statistic was "over-bloated".
The report also said that attacks in the Baga-Kawa area of northeast Nigeria on December 26 caused more than 6357 people to flee east into Chad and some 20,000 others to flee to safety within Nigeria.
Boko Haram has killed about 30,000 people and forced about two million people to flee their homes since the insurgency began in 2009.