Prosectors at the International Criminal Court have appealed an order to free Thomas Lubanga, a militia chief from the Democratic Republic of Congo (DRC), after his trial was suspended.
The court, based in The Hague in the Netherlands, ordered his release on Thursday, after the court ruled prosecutors failed to identify a key witness.
Luis Moreno-Ocampo, the chief prosecutor, appealed the court's decision on Friday in order to keep Lubanga behind bars until judges decide if the appeal has merit.
The court ruled on Thursday that Lubanga's detention was no longer fair, because the trial had been suspended due to the issue of identifying witnesses.
Last week, the court suspended his trial and criticised the chief prosecutor for abusing court processes and ignoring the judges' orders.
Moreno-Ocampo remained adamant on Friday that he would not release the name of an intermediary to Lubanga's defence team until security concerns were addressed.
"The prosecution prefers to lose the case rather than to threaten the life of a person," Moreno-Ocampo said.
He said he would release the name "as soon as protection is in place".
Lubanga is accused of enlisting and conscripting children younger than 15 for his Union of Congolese Patriots, to kill members of a rival tribe in the 1998-2003 war in the Democratic Republic of Congo.
He has pleaded not guilty and described himself as a politician, not a warlord.
The ICC is also trying other accused Congolese warlords for crimes committed during the fighting in the giant, resource-rich African country.
Lubanga's trial resumed in January, six months after prosecutors finished presenting their case.
His defence has argued that the child soldiers who testified against him made up their stories.