We know where missing girls are: Official

Nigeria's highest ranking military officer today gave a glimmer of hope to the families of more than 200 schoolgirls held by Boko Haram militants, revealing they had located the missing teenagers.

"The good news for the girls is that we know where they are but we cannot tell you," Chief of Defence Staff Air Chief Marshal Alex Badeh told reporters in the capital Abuja as the hostage crisis entered its seventh week.

Nigeria's government and military have been sharply criticised for their response to the mass abduction on April 14 and were finally forced to accept foreign help, including from the United States, in the rescue effort.

Unmanned US drones and surveillance aircraft have been scouring northeast Nigeria and neighbouring Chad from the air while British, French and Israeli teams have been on the ground providing specialist assistance.

Badeh was speaking after addressing demonstrators who had marched on Defence Headquarters in Abuja, the latest in a series of daily protests that has sought to keep up the pressure on the government.

The officer refused to divulge any further details, describing the operation as a "military secret", but he added: "We are working. We will get the girls back."

Addressing the protesters, Badeh said the military was faced with a dilemma of whether to send in ground troops, given fears of deaths and casualties among the 223 girls still being held.

"Nobody should come and say the Nigerian military does not know what it's doing. We know what we are doing. We can't go and kill our girls in the name of trying to get them back," he told the crowd.

"So, we are working. The president (Goodluck Jonathan) is solidly behind us. The president has empowered us to do the work."