Human Rights Watch on Tuesday accused Cameroon's armed forces of taking part in the killing of at least 21 civilians this month in a region where troops are battling anglophone separatists.

"Government forces and armed ethnic Fulani" carried out the slaughter in Ngarbuh, whose victims included 13 children and a pregnant woman, HRW said.

In its statement, HRW said: "Government forces and armed ethnic Fulani" carried out the slaughter in Ngarbuh, whose victims included 13 children and a pregnant woman.

The Fulani are an ethnic group also called Peuls.

"The gruesome killings of civilians, including children, are egregious crimes that should be effectively and independently investigated, and those responsible should be brought to justice," said Ilaria Allegrozzi, HRW's senior Africa researcher.

Two regions that are home to Cameroon's anglophone minority are in the grip of 29-month-old conflict between the armed forces and separatists. Rights watchdogs say atrocities and other abuses have been committed by both sides Two regions that are home to Cameroon's anglophone minority are in the grip of 29-month-old conflict between the armed forces and separatists. Rights watchdogs say atrocities and other abuses have been committed by both sides Photo: AFP / MARCO LONGARI

"Denying that these crimes have occurred adds another layer of trauma to survivors and will only embolden government troops to commit more atrocities."

The army says there were only five civilian deaths, which it said happened when fuel containers exploded in a firefight.

The incident occurred on February 14 in a remote part of the Northwest Region -- one of two English-speaking regions gripped by conflict sparked by demands for independence from majority French-speaking Cameroon.

In a separate development on Tuesday, Cameroon's constitutional council announced it had annulled the results legislative elections held on February 9 in parts of the two separatist regions.

As tensions in the anglophone regions rose, President Paul Biya, 87, vetoed appeals by moderates for a return to Cameroon's federal system As tensions in the anglophone regions rose, President Paul Biya, 87, vetoed appeals by moderates for a return to Cameroon's federal system Photo: AFP / Ludovic MARIN

HRW on Tuesday said it drew its conclusions after interviewing 25 people, including three witnesses to killings and seven relatives of victims. It also reviewed satellite imagery taken before and after the attack.

Between 10 and 15 soldiers, including members of the elite Rapid Intervention Battalion, along with at least 30 armed Fulani, entered Ngarbuh, a district of the Ntumbo village, at around 11:00 pm (2200 GMT) on February 13, it said in a statement.

After hours of looting homes and beating civilians, some of these men attacked the so-called Ngarbuh 3 district, "killing 21 civilians in four homes, then burning the houses", it said.

The military told the residents "the attack was to punish civilians suspected of harbouring separatist fighters", HRW said, quoting 12 witnesses.

cameroon Benoit Assou-Ekotto and Benjamin Moukandjo had to be separated in Cameroon's loss to Croatia. Photo: Reuters

Opposition figures in Cameroon swiftly accused the armed forces of having played a role.

But on February 17, army spokesman Colonel Cyrille Atonfack Guemo said the deaths had resulted from an "unfortunate accident" when fuel was set ablaze during a gunfight between troops and separatists after a patrol came under heavy fire.

Five civilians -- a woman and four children -- had died, and "seven terrorists" were "neutralised," Atonfack told AFP.

The following day, the UN rights office in Geneva said 23 had died in the "shocking episode" and urged Cameroon to carry out an "independent, impartial and thorough" inquiry.

The violence came less than a week after the February 9 legislative election.

On Tuesday, Cameroon's constitutional council announced it was annulling that election in the South West region's Lebialem constituency and in 10 constituencies in the North West region.

The council said the elections would have to be rerun, state television reported.

The authorities have so far released no figures on either the election results or the turnout, but the African Union has said that turnout was low across the country, particularly in the two English-speaking regions.

English-speakers account for nearly a fifth of Cameroon's population of 24 million, who are majority French-speaking.

Years of grievances at perceived discrimination against English-speakers snowballed into a declaration of independence in the anglophone regions in October 2017, which was followed by a government crackdown.

The declaration has not been recognised internationally and President Paul Biya, in power for 37 of his 87 years, has refused demands to return to a federal system.

However, the government has lately decentralised some of its powers after a "national dialogue" on the anglophone crisis which was boycotted by the separatists.

More than 3,000 people have died and at least 700,000 have fled their homes in the nearly 29-month-old unrest. Rights monitors say abuses have been committed by both sides.