Nigerian Islamist insurgent group Boko Haram has killed some 400 civilians since last year in northern Cameroon, with dozens more dying in a "heavy-handed" response by security forces, Amnesty International said in a report published Wednesday.

The report, entitled "Human rights under fire: attacks and violations in Cameroon's struggle with Boko Haram" and published in Yaounde, is based on three research missions in 2015, Amnesty said.

Two suicide bombers killed seven people on Sunday in twin attacks in northern Cameroon, an army official said, in an area near the border with Nigeria that has often been targeted by Boko Haram militants.

"According to initial information, nine people have died and 20 were injured," the official said, giving a toll that included the bombers.

At least 30 people were killed in the far north of Cameroon on Thursday in two successive suicide attacks, military and police sources said.

The first bombing took place shortly before noon in the marketplace of Kerawa, a city on the border with Nigeria, followed by a second attack about 200 meters (yards) from a military camp, said a police officer who asked not to be identified.

Boko Haram Islamists killed 10 people, including two soldiers, and torched several homes during a raid in northern Cameroon, security sources said Thursday.

Wednesday's attack in the town of Blame, near the Nigerian border, led to the deaths "of two soldiers from BIR (an elite army unit) and eight civilians," a security source said. 

Suspected Boko Haram Islamists have kidnapped 135 people in a raid on a village in northern Cameroon which left eight villagers dead, local sources said Wednesday. 

The raid took place before dawn on Tuesday in the village of Chakamari in a region known as the Extreme North, which borders Boko Haram strongholds in neighboring Nigeria.

Around 12,000 Nigerians said to have been expelled from Cameroon after fleeing Boko Haram jihadists have begun returning home, authorities in Abuja said on Wednesday.

Nigeria's National Emergency Management Agency (NEMA) said it had received 1,121 of the returnees at the Sahuda border post in northeastern Adamawa state.

A female suicide bomber travelling on a tricycle blew herself up Friday at a market in the northeast Nigerian city of Maiduguri, witnesses said, as Nigeria and its neighbors finalize a force to combat Boko Haram jihadists.

"There had been a suicide attack on Gamboru market this morning. It was a female suicide bomber. The attack happened around 6:30 am (0530 GMT) as the grocers were arriving in the market which starts early," said Babakura Kolo, a vigilante helping the Nigerian army combat Boko Haram jihadists in the region.

Nigerian President Muhammadu Buhari arrived in neighboring Cameroon on Wednesday for talks on how to combat the escalating regional threat from Boko Haram Islamists.

Security was tight for the 24-hour visit, after a surge of Boko Haram violence in Cameroon including an unprecedented series of five suicide bombings in the far north.

Cameroon will send around 2,000 extra troops to the north of the country to fight Boko Haram, the Nigerian jihadist movement behind bloody cross-border raids and suicide bombings, state television said Tuesday.

"Almost 2,000 extra soldiers will be deployed in the Extreme North region," on the border with northeastern Nigeria, according to the report on Cameroon Radio-Television, which gave no timetable for the operation.

Three villagers were beheaded during a weekend raid in Cameroon by Boko Haram jihadist fighters from neighboring Nigeria, security forces said Monday.

The attacks took place on Sunday in the neighboring villages of Tchebe-Tchebe and Dzaba, about 10 kilometers (six miles) from the Nigerian border, said a security agent based in the region.