Nigeria’s highest ranking military officer on Monday gave a glimmer of hope to the families of more than 200 schoolgirls held by Boko Haram terrorists, 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 Defense Staff Air Chief Marshal Alex Badeh told reporters in the capital Abuja, according to the AFP news agency.

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

Unmanned U.S. drones and surveillance aircraft have been scouring northeast Nigeria and neighboring 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 Defense 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,” Badeh said, according to AFP.

Concerns over the fate of the girls were heightened after Boko Haram’s leader threatened to sell them as slaves. The Al-Qaeda-linked group has publicized a video showing some of the kidnapped girls, claiming they had converted to Islam.

The mass-kidnapping is not the group’s first attack on a school. Just two months previously the group shot and burned to death dozens of students in an attack on a boarding school.

In a similar attack in 2013, Boko Haram terrorists murdered 50 students as they slept in their dormitories. In July of that year 29 pupils and a teacher were burned alive in another school.