She rejected reports from Belgrade on Wednesday that Mladic had fled to Russia to evade attempts to capture him and transfer him to the war crimes tribunal in The Hague, Netherlands.
"Mladic is in Serbia and, as you know, Mladic is protected by power of the army," Del Ponte told reporters. "What I need is an... obligation from Belgrade to arrest him and deliver him to The Hague."
Del Ponte said her assertion was based on information she had received, but she did not elaborate or offer other details.
She also said she wanted to start the trial of Mladic in July along with the other eight suspects charged with genocide during the 1992-95 war in Bosnia.
Both Mladic and the other top war crimes suspect, Radovan Karadic, another former Bosnian Serb leader, have been sought for more than a decade by the U.N. court, indicted for the 1995 massacre of about 8,000 Bosnian Muslim men and boys in Srebrenica – Europe's worst carnage since World War II – and for other war crimes during the war.
Del Ponte's remarks came after she met with the European Union's commissioner in charge of expansion, Olli Rehn, and other officials to ask for the EU's help in getting Belgrade's full cooperation with the tribunal.
Rehn warned that the Belgrade's path to closer relations with the EU and possible membership in the bloc could be frozen if it does not take action.
In particular, he said the EU could halt talks with Serbia on a so-called stability and association agreement, which would give Serbia much needed financial aid to boost economic and political reforms to ready it for possible EU membership.
"Suspension of negotiations is certainly one alternative and I expect that the Serbian government will take this message very seriously," Rehn said. "Serbia has to choose now between its nationalist past and a European future."
Del Ponte also held talks with EU foreign policy chief Javier Solana and NATO's Secretary General Jaap de Hoop Scheffer to push for the two organizations to put more pressure on Serbia to hand over the fugitives.
In Belgrade, Serbia-Montenegro Defense Minister Zoran Stankovic said his government's search for Mladic was ongoing, adding he recently met with the fugitive's wife and son.
"The operation to catch Mladic is under way and I cannot comment on it," Stankovic told reporters. "All available army personnel are engaged in this."
Stankovic offered no details on his meeting with Mladic's wife and son at the Defense Ministry building in Belgrade shortly before the New Year.
Stankovic announced an intensified search for Mladic last month and accepted the military was partly to blame for the failure to arrest him.
By Constant Brand