* Most of the victims had their throats slit
* Boko Haram attacks on civilians intensifying
* Military offensive has yet to defeat Islamists
DAMATURU, Nigeria, Oct 20 (Reuters) - Islamist Boko Haram militants disguised as soldiers killed 19 people at a checkpoint on Sunday, mostly by slitting their throats, witnesses said.
Survivor Adamu Mallam, a trader, was hauled out of his vehicle and saw two other people shot dead by men dressed in military uniforms.
"They made me lie face down on the ground. I was next to be killed," he told Reuters by telephone, voice trembling. "I heard a man close to me screaming. They slaughtered him with a knife."
One of the attackers received a phone call and they all rode off on motorbikes, Mallam and another trader said. They set five trucks ablaze during the assault, Mallam said.
"I suspect they were alerted to a security presence so had to flee. That call saved our lives," said the other trader, who asked not to be identified for fear of reprisals.
Militant group Boko Haram has been fighting for more than four years to try to revive old Islamic kingdoms in religiously mixed Nigeria. It remains the top security threat to Africa's leading energy producer despite an all-out military offensive against it ordered by President Goodluck Jonathan in May.
The offensive involves air strikes, ground attacks and some co-opting of civilians to form youth militias that can seek out Islamist suspects. It has pushed them out of bases in the forests and deserts of the remote northeast, where they were on the verge of establishing a de facto Islamic state.
But a spate of vicious reprisal attacks have demonstrated that the insurgents are far from defeated and they have begun using the new tactic of fake checkpoints to spread more fear.
Suspected Boko Haram gunmen killed 159 people in two roadside attacks in northeast Nigeria last month, also involving fake checkpoints.
The group has stepped up the number and scale of attacks in the past two months, with hundreds of killings reported, many of them more targeted towards civilians than in the past.
In one of the most harrowing last month, suspected Boko Haram fighters stormed a college in northeastern Nigeria and shot dead around 40 male students.
As the conflict in the northeast intensifies, both sides are being accused of atrocities. Amnesty International reported last week that around 1,000 people, mostly Boko Haram suspects, had died in Nigerian jails in the first half of the year.
Some starved to death. Others died after being shot or badly beaten then getting no medical attention.
The government says detainees are well treated and rare cases of abuse dealt with. (Reporting by Joe Hemba; Editing by Alison Williams)