It was the latest in a spate of attacks blamed on the Nigeria-based militant group in northern Cameroon, which have driven tens of thousands of people from their homes.
The security source said the attacker blew himself up in the town of Djakana near the Nigerian border overnight.
Twenty-five dead in suicide bomb attack on north Cameroon market
"Seven people were killed immediately, including the bomber. Four others later succumbed to their wounds", the security source said.
"Others who were wounded are in hospital. We fear the number of victims could rise."
Most of the victims were members of a local vigilante group tasked with hunting down Boko Haram fighters.
"They were gathered in a video room when the attacker entered and triggered his explosives," the security source said.
Boko Haram's seven-year insurgency has left at least 20,000 people dead in Nigeria and border areas of neighbouring Niger, Chad and Cameroon, and made more than 2.6 million homeless.
Some 2,000 Chadian soldiers are set to launch a counter-offensive against the group in the region, as part of a fight back by the four countries targeted by the group.
Boko Haram has regularly used women and children to stage suicide bombings, targeting mosques, markets, bus stations and checkpoints.
But the overnight attack comes after a lull in violence in this border zone near Nigeria.
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The UN humanitarian coordinator for Cameroon warned this month that unabated attacks by the jihadists had sparked food insecurity and driven 190,000 people from their homes thereby creating a fertile ground for recruitment by Boko Haram.
Najat Rochdi had told AFP in an interview that Boko Haram members were attacking villages and food supply routes as well as burning homes and fields across northern Cameroon on a daily basis.
She said that in the last six months alone, the number of Cameroonians displaced within their own country had jumped from 60,000 to 190,000.
In addition, Cameroon is hosting 60,000 refugees from Nigeria and another 312,000 from the Central African Republic, amounting to more than 500,000 displaced people in all.
The number at risk of going hungry has soared from 900,000 to 2.4 million since January.
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"It is a kind of silent crisis, which is really the danger," Rochdi said, warning that if humanitarian needs are not addressed in Cameroon, "we will see a radicalisation" of young people in the country.
"If people are not left with some hope, the only alternative for them is Boko Haram," she cautioned.
There is a gaping budgetary gap with only 30 percent of the requested $280 million (248-million-euro) humanitarian aid budget for Cameroon this year funded so far.
According to the UN, some 250 children recruited or abducted by Boko Haram in Cameroon have meanwhile managed to escape over the past nine months, according to the UN.
Some of them were girls who had been raped daily.
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