Nigeria's military has located nearly 300 schoolgirls abducted by Islamic extremists, but fears using force to try to free them could get them killed.
The country's defence chief, Air Marshal Alex Badeh, told demonstrators who were supporting the much criticised military that Nigerian troops could save the girls.
But he added: "We can't go and kill our girls in the name of trying to get them back."
He spoke to thousands of demonstrators who marched to Defence Ministry headquarters in Abuja, the capital.
Many were taken in on buses, indicating it was an organised event.
Asked where the girls were located, Badeh refused to elaborate. But he told the crowd: "We want our girls back.
"I can tell you we can do it. Our military can do it. But where they are held, can we go with force?"
The crowd roared back: "No!"
"If we go with force what will happen?" Badeh asked.
They will die," the demonstrators responded.
That appeared to leave negotiation the sole option, but a human rights activist close to negotiators said a deal to swap the girls for detained Boko Haram members was agreed and then scuttled at the last minute by President Goodluck Jonathan.
The activist - who is close to those mediating between Boko Haram extremists and government officials, but wished to remain anonymous - said the girls would have been freed on Monday of last week.
But Jonathan had already told British officials that he would not consider an exchange.
Nigeria's military and government have faced national and international outrage over their failure to rescue the girls seized by Boko Haram militants from a remote northeastern school.
More than 300 teenagers were abducted in the town of Chibok on April 15.
Police say 53 escaped on their own and 276 remain captive.
A Boko Haram video has shown some of the girls reciting Koranic verses in Arabic and two of them explaining why they had converted from Christianity to Islam in captivity.
Unverified reports have indicated two may have died of snake bites, that some have been forced to marry their abductors and that some may have been carried across borders into Chad and Cameroon.
Boko Haram - the nickname means "Western education is sinful" - believes Western influences have corrupted Nigerian society and want an Islamic state under strict sharia law.
But the population of 170 million people is divided almost equally between Christians and Muslims.
Jonathan was forced this month to accept international help in the hunt for the girls.
American planes have joined the search and Britain, France, Israel and other countries have sent experts in surveillance and hostage negotiation.
Jonathan's reluctance to accept offered help for weeks is seen as unwillingness to have outsiders looking in on what is considered a very corrupt security force.
Soldiers have said they are not properly paid, are dumped in dangerous bush areas with no supplies and that the Boko Haram extremists holding the girls are better equipped than they are.
Some soldiers have said officers enriching themselves from the defence budget have no interest in halting the five-year-old uprising that has left thousands dead.
Earlier this month, soldiers said to be near mutiny fired on the car of a commanding officer after 12 soldiers were killed by the insurgents in an ambush.
The military also is accused of killing thousands of detainees held illegally in their barracks.
Some were shot, some of them tortured and many starved to death or asphyxiated in overcrowded cells.