Nigeria's military has located nearly 300 school girls abducted by Boko Haram almost seven weeks ago, the country's chief of defence said.
Air Chief Marshal Alex Badeh, Nigeria's Chief of Defence Staff, said on Monday that any potential armed rescue operation was fraught with danger as the 223 girls still held hostage could be caught in the crossfire.
Boko Haram fighters kidnapped 276 girls from the remote northeastern town of Chibok on April 14, leading to global outrage.
"The good news for the girls is that we know where they are but we cannot tell you," Badeh told reporters in the capital Abuja, as the hostage crisis entered its seventh week.
"We can't go and kill our girls in the name of trying to get them back," he said.
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Nigeria's government and military have been sharply criticised for their slow response to the mass abduction and were finally forced to accept foreign help in the rescue effort.
US drones have been surveying northeast Nigeria and neighbouring Chad from the air while British, French and Israeli teams have been on the ground providing specialist assistance.
The military has previously said that the search was centred around the Sambisa forest area of Borno, in northeast Nigeria, where makeshift fighter camps have previously been found as well as arms and ammunition caches.
On Monday, gunmen killed four Nigerian soldiers in an ambush on a military patrol in central Plateau state, about 180kms southeast of Jos, a local government official said.
It was not immediately clear if Boko Haram had carried out this attack.
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Since the girls were captured, according to a Reuters count, at least 470 civilians have died violent deaths in various locations at the hands of Boko Haram, which says it is fighting to establish an Islamic state in religiously mixed Nigeria.
Nigerian President Goodluck Jonathan has branded the group an "al-Qaeda of West Africa".
The United Nations Security Council last week designated Boko Haram an al-Qaeda-linked organisation, in a move designed to curb any overseas funding and support, as well as restrict its leaders' movements.
But analysts have questioned whether the sanctions would have any effect on the ground, given the group's largely localised campaign of murderous violence to create an Islamic state in northern Nigeria.