Nigerian Insurgents Expanding 'Islamic Caliphate' As Troops Fight Back

Haram takes over string of northeast towns

* Adult males are first to be slaughtered - women survivors

* Killings followed by preaching of virtues of Islamic rule

* Leader proclaimed "Muslim territory" at Gwoza

* Nigeria army trying to win back lost ground

(Reuters) Islamist Boko Haram militants carrying automatic rifles and machetes roar into northeast Nigerian towns or hamlets in columns of pickup trucks and motorbikes and fire at all adult men they see. They often finish them off with knives.

When they have hoisted their black flag inscribed with Koranic verses over government offices or the local emir's palace, they tell the surviving women they will marry them and "live in peace", according to survivors who escaped from the town of Gwoza, seized by Boko Haram last month.

Escapees from other seized northeast towns, such as Madagali, say the Islamists also torch churches, and force Christian women to convert to Islam under pain of death.

Slaughtering and preaching a "better life under Islamic rule", the jihadist group has taken over a string of locations in recent weeks in Nigeria's remote northeast, in what looks like a plan to seize and hold a "Muslim territory" or caliphate, apparently inspired by the Islamic State (IS) in Iraq and Syria.

In Gwoza last month, near the Cameroon border, Boko Haram's leader Abubakar Shekau declared such an Islamic-ruled enclave. He made the proclamation in a video showing his fighters running amok, firing in all directions and killing captives. They rode in pick-up trucks and captured army vehicles, including at least one armoured car.

The conquests are a departure from Boko Haram's usual hit-and-run tactics. Shekau is believed to be mimicking the IS proclamation of its caliphate which caused alarm in the Middle East and West amid fears of an upsurge in jihadist attacks.

With memories still present of the 1967-70 Biafra secession war, Nigeria's armed forces have vowed to resist any takeover of territory in Africa's No. 1 oil producer by Boko Haram, whose advances have even threatened the Borno state capital Maiduguri.

But refugees often speak of soldiers fleeing with them from the assaults by the militants' mobile columns, which can consist of hundreds of well-armed fanatical fighters.

When they appear, the killing often seems indiscriminate.

Gwoza schoolgirl Indiyanatu Musa, 16, who witnessed the Aug 5 attack on the town with her school friends, said the raiders started shooting as soon as they arrived at a local park.

"They said 'Shekau sent us. You are condemned to death be you Christian or Muslim,'" she told Reuters at Gombe in the neighbouring state of the same name, to where she had fled with her sisters after escaping from Gwoza in Borno state.

"Within a short time, the whole park was filled with bodies just lying everywhere. I was screaming and so were my schoolmates and the rest of the women around," said Musa.