The leader of Nigeria's extreme Islamist Boko Haram group has called on followers to torch more schools and murder teachers after 46 students were gunned down by militants in an attack on a dormitory.

Abubakar Shekau said in a video message that Western-style education was a "plot against Islam."

"Teachers who teach Western education? We will kill them. We will kill them in front of their students, and tell the students to henceforth study the Koran," he said.

"We are going to burn down the schools if they are not Islamic religious schools for Allah."

Shekau denied that his fighters killed children. "Our religion does not permit us to touch small children and women. We don't kill children," he insisted and dismissed claims that a ceasefire had been agreed with the government.

In recent weeks the group, whose name means "Western education is forbidden" in the Hausa language, has increased its attacks on "soft" civilian targets after the government launched a military crackdown.

In April a state of emergency was declared in the northeastern states of Andamaye, Borno and Yobe, where Boko Haram is most active, with thousands of troops deployed to root out fighters.

Stream of recruits

According to official figures, 300 classrooms have been torched by the group since 2009 and the campaign against schools has intensified.

In June, 16 students were killed in consecutive attacks on secondary comprehensive schools in Yobe and Borno. An attack on a university left 16 dead in April.

Many of the students killed in the most recent attack were gunned down attempting to flee when the building was set alight with them inside.

Experts believe that attacks on schools serve two purposes for the group: halting the spread of Western influence and generating a stream of recruits from children pulled out of school by fearful parents.

Some children are said to be inducted into the group after being given dates stuffed with the horse tranquiliser Tramadol and kidnapped.

A gardener said he had fled his home village with his family to the capital Lagos after Boko Haram set up camp nearby.

He told the Guardian that one night he had been awoken by his neighbour wailing. "[Boko Haram] told her they took her son to their camp to fight for Allah," he said. "They said the boy's family is now the Boko Haram. My wife said we should leave that very day."

A soldier in Damaturu told the newspaper: "As they [children] are no longer in school, their parents send them to sell groundnuts or boiled eggs on the long queues of vehicles created by the stop-and-search. We know they are paid by the Boko Haram to spy on us."