GOMBE Nigeria (Reuters) - First comes the killing, then the preaching.
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 armored 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 neighboring 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.
Residents from Gamborou-Ngala, also taken over by the group last month, have spoken of similar widespread slaughter.
Often, the survivors say, the killing goes on for days, with the "Boko Haram boys" going from house to house, hunting for males aged 18 or over and dragging them out to be shot.
"I saw several boys who had been hiding in the ceilings of their parents' houses brought out and shot," Musa said.
Males over 18 were targeted because Boko Haram believed they were members of a pro-government vigilante organization, the Civilian JTF, which Shekau has threatened. "We warn the vigilante, called Civilian JTF ... while you are shouting for mercy, we will strike, smash your heads and kill you all," he said in a video released Aug. 24 claiming Gwoza's takeover.
Women were spared. "They told us that they would not harm us, but that they are only killing our men because our men form vigilantes to fight them," said Aisha Abubakar, who managed to escape from Gwoza with her four children and husband.
The fighters who took the town also pillaged shops and stores, Musa said.
When the killing had ended, remaining inhabitants were expected to attend Islamic prayers. "Each morning, they go around the houses knocking and calling on us to go for morning prayers and a sermon in the mosque," Musa said.
In scenes reported elsewhere too, Gwoza residents say they saw black flags flying over the Emir's palace carrying inscriptions in Arabic and the local Hausa language which read "Allahu Akbar" (God is Great).
At Gamborou-Ngala, which is also on the border with Cameroon, Boko Haram fighters had occupied the immigration and customs office, opening fire if threatened, said Alice Adejuwon, one of hundreds of Nigerians who fled over the frontier.
Boko Haram's seizure of northeast towns and territory outside of its Sambisa forest and Mandara Mountains hideouts has alarmed both Nigerians and the country's western allies. The top U.S. diplomat for Africa, Linda Thomas-Greenfield, last week called the group's recent gains a "sober reality check".
"Unless swift action is taken, Nigeria could be facing a rapid takeover of a large area of its territory reminiscent of ISIS's lightning advances in Iraq," the Nigeria Security Network said in a Sept 2 report. The Network groups Nigerian and international experts working on that country's security issues.
"They are beginning to operate like a conventional army ... They are reported to be using armored vehicles, including tanks, and heavy weapons," the Network reported.
Boko Haram has killed thousands since it launched its anti-government insurgency in 2009, mostly in the poor northeast. Under Shekau's leadership it has appeared to become increasingly violent, carrying out shootings and bombings across the north, and even, sporadically, in the central federal capital Abuja.
Stung by the northeast losses, Nigeria's armed forces have counter-attacked with warplanes. They halted the group's advance at Bama southeast of Maiduguri, and are also fighting to block a Boko Haram push into neighboring Adamawa state.
At Gwoza last month, government soldiers escaped to the rocky hills around the town along with the fleeing civilians.
"I saw three soldiers in the hills, there was one who called his superior for reinforcement, they told him that they should just look for a way to escape," said Bukar Bulama, 26, a Gwoza market trader who escaped with a friend Mohammed Usman, 23.
"I also saw the Emir of Gwoza, he was in the hills too," said Bulama.
Back in the Emir's palace, Shekau had installed himself, promising "a better life under Islamic rule," said Musa.
Abubakar said, before she managed to escape, she was among a group of Gwoza women initially taken to see him to ask for the killings to stop. "We went to the Emir's palace and saw a man sitting in the Emir's seat dressed in our Emir's royal clothes with a turban. He said he was Shekau".
He told them they would not be hurt. With their men folk dead, militants would marry them, "we will all live in peace".
"He also said: 'We have installed an Islamic Caliphate in Gwoza so every one of you must comply or be killed," she said.
Counter-terrorism experts say some links exist between Boko Haram and other Islamist groups, such as al Qaeda's North African franchise and Somalia's al Qaeda-linked al Shabaab, but there is little evidence so far of close extensive cooperation.
In a national vote due in February, Nigeria's president, southerner Goodluck Jonathan, is expected to seek re-election. Many believe political tensions stemming from the historic rivalry between Nigeria's mostly Muslim north and largely Christian south are also stoking the Boko Haram insurgency.
Thousands of northeast residents have fled southwards before the marauding militant columns, fearful of being caught in the group's rolling campaign of killing and preaching Islamic rule.
"Better to submit to Allah before it becomes too late," Shekau said in last month's video.
Additional reporting by Pascal Fletcher in Lagos and Lanre Ola in the northeast; Writing by Pascal Fletcher; editing by Anna Willard