By Lanre Ola

MAIDUGURI, Nigeria (Reuters) - The latest big Islamist attack in Nigeria has killed least 125 people, police said on Wednesday after gunmen rampaged through a town in the northeast, near the Cameroon border.

Details emerging of the scale and ferocity of Monday's massacre in Gamburu underscore how far Nigerian security forces are from protecting civilians in a region where U.S. experts are preparing to help find and free 200 abducted schoolgirls.

Scores of gunmen whom police suspect were from Boko Haram, the al Qaeda-linked group that seized the girls in the same region last week, surrounded Gamburu before dawn on Monday. They sprayed automatic gunfire around the market town, which was crowded with traders gathering before the heat of the day.

Witnesses said they burned down houses and, in some cases, slit people's throats. A police officer assessing the scene on Wednesday said the death toll had reached at least 125.

Demanding an Islamic state, Boko Haram has been fighting in the northeast for five years but attracted renewed global attention last month with the abduction of girls taking exams in the village of Chibok, also in the south of Borno state.

This week, the United States said Nigeria had accepted an offer of military and civilian experts to locate and recover the 200 or so captives. Britain is also sending a small team and France has also offered assistance.

The international attention has added to pressure on the government to show it is working to protect civilians. Police offered a $300,000 reward on Wednesday for tip-offs, listing six phone numbers for anyone with "credible information" to call.

The kidnappings and numerous other attacks by Boko Haram have overshadowed Nigeria's hosting of a World Economic Forum, starting in the capital Abuja on Wednesday. Nigerian officials had hoped the event would draw attention to the potential of Africa's biggest economy as an investment destination.

"HORRIBLE"

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A witness to the Gamburu attack, Talatu Sule, said she survived by hiding at home with her children. Afterwards, she went out with the police team to see the devastation.

"I counted 85 dead before I lost interest in counting. This is horrible," she told Reuters by telephone. "They burned vehicles, cars and 17 trailers loaded with cows and grains in the market."

A police source said there may be more bodies beyond the town in the bush or in the rows of charred houses.

Public anger mounted after locals on Tuesday said another eight girls had been seized from the same remote northeastern area by suspected members of the group.

Boko Haram leader Abubakar Shekau has threatened in a video to sell the girls abducted on April 14 from a secondary school in Chibok "on the market".

Last month's kidnapping occurred on the day a bomb blast, also claimed by Boko Haram, killed 75 people on the outskirts of Abuja, the first attack on the capital in two years. Another bomb in roughly the same place killed 19 people last week.

President Goodluck Jonathan welcomed the U.S. offer to send an American team to Nigeria to support the government's efforts to find the girls.

U.S. President Barack Obama said the kidnappings "may be the event that helps to mobilize the entire international community to finally do something against this horrendous organization that's perpetrated such a terrible crime".

(Additional reporting by Camillus Eboh; Writing by Tim Cocks; Editing by Andrew Heavens, Giles Elgood and Alastair Macdonald)