* Aid agencies say 100 Nigerian soldiers killed since Dec. 26
* Army says death toll lower than that
* More than 6,357 people forced to flee to Chad
* Security is a campaign issue ahead of Feb. 16 election
* U.N. official warns attacks disrupting aid distribution (Adds army spokesman disputing death toll)
By Tom Miles
GENEVA, Jan 18 (Reuters) - Armed groups have killed more than 100 soldiers and seized a huge stock of weapons in clashes in northeast Nigeria since Dec. 26, a report by a group of aid agencies said on Friday, weeks ahead of an election in which security is a campaign issue.
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, the report said.
They 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 Feb. 16 in which President Muhammadu Buhari is seeking a second term, and his critics have seized upon security as an issue.
Buhari, a retired general and former military ruler, came to power in 2015 promising to defeat the Boko Haram insurgency, which aims to create an Islamic state in the northeast. Islamic State in West Africa (ISWA) split from Boko Haram in 2016 and has carried out most attacks in the last few weeks.
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."
"Undoubtedly we recorded some casualties in these encounters, since we are at war and not a picnic, but certainly not the over-bloated casualty statistics being touted by UNHCR," said Nigerian Army spokesman Onyema Nwachukwu in a text message.
The report said attacks in the Baga-Kawa area of northeast Nigeria on Dec. 26 caused more than 6,357 people to flee east into Chad and some 20,000 others to flee to safety within Nigeria.
Last month, ISWA seized the town of Baga, where a multi-national force formed by Nigeria, Chad, Cameroon and Niger to fight the militants is based. The Nigerian army later regained Baga and said it had given ISWA a "bloody nose".
Boko Haram has killed about 30,000 people and forced about two million people to flee their homes since the insurgency began in 2009.
ISWA has been responsible for attacks on military bases of recent weeks. In contrast, Boko Haram typically carries out suicide bombings and gun raids, though it said it carried out Monday's attack on the northeast Nigerian town of Rann.
United Nations Humanitarian Coordinator in Nigeria, Edward Kallon, said on Friday that repeated attacks were "severely affecting" aid delivery to the 76,000 internally displaced people living in Rann.
Boko Haram held territory in northeast Nigeria around the size of Belgium in early 2015 but was forced out of most of that land by troops from Nigeria and neighbouring countries.
"When this administration came, Boko Haram was holding 17 local governments in the northeast. They are not holding any local governments," said Buhari in a televised interview on Wednesday, when asked about his security record in light of recent attacks.
(Reporting by Tom Miles; Additional reporting to Camillus Eboh in Abuja; Writing by Alexis Akwagyiram Editing by Angus MacSwan and Toby Chopra)
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