More than 1,000 people accused of supporting Boko Haram in Cameroon are being detained in military bases and prisons, often without any evidence, and dozens are dying from disease, malnutrition and torture, a rights group said on Thursday.

Since a regional offensive last year drove Boko Haram from most of their strongholds, the Islamist militants have waged a guerrilla-style campaign targeting civilians. In Cameroon, teenage girls have killed dozens in suicide bombings carried out by the group.

But a crackdown by the government and security forces on the Islamist militants has fuelled the widespread abuse of civilians across Cameroon's Far North region, said Amnesty International.

Cameroonian government officials were not immediately available for comment.

"We are not necessarily talking about Boko Haram fighters - but about normal people who happened to be in the wrong place in the wrong time," said Amnesty researcher Ilaria Allegrozzi.

"In the fight against Boko Haram, innocent people are paying the price," she told the Thomson Reuters Foundation.

Many people are held at illegal detention sites in military bases run by Cameroonian troops before being transferred to official prisons, Amnesty said in a report.

Several detainees in the military bases told Amnesty that they were tortured and beaten with sticks, whips and machetes, sometimes until they lost consciousness.

"Two prisoners were beaten up so badly that they died in front of us," Amnesty quoted a 70-year-old man as saying. "The men kicked them, slapped them violently, and hit them with wooden sticks."

In the main prison in Maroua, the capital of the Far North region, between six and eight people die every month in dirty and overcrowded cells where malnutrition is rife, Amnesty said.

Detainees suspected of supporting Boko Haram who are brought to trial risk being convicted and sentenced to death, despite there often being little or no evidence, the rights group said.

More than 100 people have been sentenced to death in Maroua's military court since July 2015, although none have yet been executed, said the "Right Cause, Wrong Means" report.

Most defendants are charged under an anti-terrorism law passed in 2014, which is ambiguous and vague, Amnesty said.

A 27-year-old who was arrested after sending a text message to his friends, joking about Boko Haram recruiting graduates, could face the death penalty, the rights group said.

"If a student can face the death penalty for sending a sarcastic text message, it is clear that there is a serious problem with the design and use of Cameroon's anti-terrorist legislation," said Alioune Tine, Amnesty's regional director.

More than 15,000 people have been killed and 2.4 million uprooted in Nigeria and neighboring Chad, Niger and Cameroon during Boko Haram's seven-year campaign to carve out an Islamic caliphate in northeast Nigeria.