Boko Haram fighters have shot or burned to death about 90 civilians and wounded 500 in ongoing fighting in a border town near Nigeria, officials in Cameroon have said.

Some 800 Islamic extremists attacking the town of Fotokol “burned churches, mosques and villages and slaughtered youth who resisted joining them to fight Cameroonian forces”, the information minister, Issa Tchiroma Bakari, said on Thursday. The Nigerian insurgents also looted livestock and food in the fighting that began on Wednesday.

Boko Haram has been using civilians as shields, making it difficult to confront them, although reinforcements have arrived in Fotokol, according to a military spokesman.

Hundreds of insurgents were killed on Wednesday along with 13 Chadian and six Cameroonian troops, the defence minister, Edgard Alain Mebe Ngo, said. At least 91 civilians have been killed and most of the more than 500 who have been wounded cannot be taken quickly to hospital, he said. There was no way immediately to confirm the account independently.

The fighters are believed to have crossed into Cameroon from nearby Gambaru, a Nigerian border town that had been an extremist stronghold since November but that was retaken this week. The fighters were driven out by Chadian and Nigerian air strikes supported by Chadian ground troops.

African Union officials were finalising plans on Thursday for a multinational force to fight the spreading Boko Haram uprising, though there are questions about funding. Last week the AU authorised a 7,500-strong force from Nigeria and its four neighbours, Cameroon, Chad, Niger and Benin.

Senior officers from the UN peacekeeping department were attending the meeting in Yaounde, Cameroon’s capital, a UN official said. The Africans want UN security council approval and money to fund the mission, added the official, who spoke on Wednesday at the UN and requested anonymity because he was not authorised to speak to the press about the meeting.

President François Hollande said France was providing support with weapons, logistics and operations for the multinational effort. At a news conference in Paris, he stopped short of saying whether France was involved in military action. The country has a large air base at N’Djamena, the capital of Chad, which will lead the multinational force.

International concern has grown as Boko Haram has increased the tempo and ferocity of its attacks, just as Nigeria is preparing for presidential and legislative elections on 14 February.

An estimated 10,000 people were killed in Boko Haram violence last year compared with 2,000 in the first four years of Nigeria’s Islamic uprising, according to the Council on Foreign Relations.