"Only four of those accused remain at large, including [Radovan] Karadzic and [Ratco] Mladic," Carla Del Ponte told a hearing of the European Parliament's international affairs committee.
The Bosnian Serb leaders Mladic and Karadzic are accused of genocide over the 1995 Srebrenica massacre of 8,000 Bosnian Muslims, Europe's worst single atrocity since World War II. Mladic is believed to be hiding in Serbia, while the whereabouts of Karadzic remain unknown.
"We don't have Karadzic and Mladic in custody. If they are not arrested and tried by the tribunal, that will be the biggest failure ... of the international community," Del Ponte said.
The UN chief war crimes prosecutor said the European Union should make the arrest of Mladic a condition for signing an agreement on closer ties with Serbia.
"There should be no signing of the stabilization and association agreement (SAA) before Ratko Mladic is under arrest and full cooperation [with the U.N. war crimes tribunal] is established," Del Ponte said.
The SAA is a treaty that would pave the way for Serbia's membership to the European Union.
Since it was set up in 1993, the Hague Tribunal has brought charges against 161 officials suspected of committing war crimes and crimes against humanity in the former Yugoslavia, Del Ponte said.
"[Legal] procedures have already been completed for 106 of those charged," she said, adding that 56 were already serving prison sentences.