Islamic State fills the void in Nigeria as soldiers retreat to 'super camps'

* Nigerian army to establish 'super camps' in northeast

* Soldiers leaving some towns they protected previously

* New strategy affecting access by aid workers to some areas

By Paul Carsten

ABUJA/MAIDUGURI, Reuters, Sept 16 (Reuters) - When Islamic State gunmen stormed the northeast Nigerian town of Magumeri on the night of August 21, they had free rein.

Nigerian soldiers had left the town earlier that month under a new strategy of withdrawing to "super camps" that can be more easily defended against insurgents the army has been struggling to contain for a decade.

Unchallenged, the Islamist militants torched a clinic in Magumeri, ransacked government buildings and looted shops before returning to another town they had raided that night called Gubio, residents said.

The new military strategy announced by President Muhammadu Buhari in July to concentrate soldiers in big bases is designed to give troops a secure platform from which they can respond quickly to threats in the region and raid militant camps.

People familiar with the military's thinking and security officials, however, say the new tactic for fighting Islamic State's West Africa branch and Boko Haram is mainly an attempt to stem casualties.

The military did not respond to requests for more details about its strategy or the impact it will have on the region.

"We strongly believe the days of BH (Boko Haram) moving freely and passing in between static defensive locations are over," Major General Olusegun Adeniyi, who commands the anti-insurgency operation, told reporters last month.

Boko Haram launched an insurgency in 2009 to overthrow the government and establish an Islamic caliphate. The group, whose unofficial name means "Western education is forbidden", held territory the size of Belgium in 2014 but a multinational offensive recaptured much of it the following year.

The group split in 2016 and the faction that has been the greater threat ever since won the recognition of Islamic State.

The decade of war has killed more than 30,000 civilians and spawned what the United Nations calls one of the world's worst humanitarian crises, which foreign nations are trying to contain with billions of dollars of aid.

But the crisis shows no sign of abating.

'IT'S A MESS'

The army's withdrawal into large bases has coincided with a string of insurgent raids on newly unprotected towns and has left the militants free to set up checkpoints on roads as they roam more freely across the countryside, according to three briefing notes from an international aid and development organisation, two security officials and residents.