* Islamists blow up and set fire to masts in north
* MTN, Etisalat, Airtel all targeted
* Seven members of Boko Haram killed in Maiduguri - military
By Camillus Eboh and Joe Brock
ABUJA, Sept 7 (Reuters) - Nigerian Islamist sect Boko Haram on Friday claimed a spate of attacks on mobile phone masts belonging to MTN, Airtel, Etisalat and two private operators, accusing the firms of helping the government spy on them.
Boko Haram, a Taliban-style sect that says it wants to impose sharia or Islamic law on religiously mixed Nigeria, has killed hundreds this year, mostly in the largely Muslim north.
In the past few days, attackers have blown up, burnt or torn down several mobile telecoms installations in the north of the country. On Friday, police officers were guarding telecoms infrastructure across Nigeria as a precaution.
"The Movement ... is fighting to establish an Islamic state and we will wage war against anyone who tries to stop us," Abul Qaqa, a spokesman, said in a written statement issued from the group's headquarters in the northeastern city of Maiduguri.
"This is the reason we attacked the telecoms firms, because they are providing the security personnel with information used to track our members. We will continue attacking them until they stop," he said.
Several sect members had been arrested due to information gleaned from their mobile phones, he added. The group has been weakened by several arrests and the killing of some of its fighters.
Boko Haram usually targets members of the security forces, government offices and churches. Its low level insurgency has lasted more than two years.
Qaqa also reiterated a threat against the media which the sect accuses of being biased against it. It has attacked the media in the past, blowing up the offices of ThisDay, a local daily newspaper.
The group would continue to attack schools, in retaliation for raids on its own "Islamic schools" by security forces, Qaqa added.
Separately, the military joint task force for the northeast of the country said on Friday that its personnel had killed seven Boko Haram members and arrested 13 more on Wednesday after a gun battle in the northeastern city of Maiduguri, the group's heartland.
The Inspector-General of Police earlier ordered all regional forces to set up special units "to ensure the safety of telecommunication equipment and installations".
In the latest reported attack, gunmen on motorbikes used high-calibre explosives to destroy masts belonging to MTN, Etisalat and privately-owned companies Globacom and Visafone in the north's biggest city Kano on Thursday, police said.
Attackers also wrecked more than a dozen masts in Kano and Maiduguri on Wednesday.
A spokeswoman for MTN Nigeria said by email on Friday that the operator was trying to restore services to customers in the areas hit by the sabotage.