The rumors from Belgrade about Mladic's arrest are false and completely groundless, del Ponte told a news conference in The Hague.
"There is no indication at all that negotiations about his surrender are currently under way," she told the press, adding that the Belgrade government had assured her that Mladic remained at large.
"Ratko Mladic is in Serbia, there is no doubt about this. He has been there since 1998. During all this time he has been, and he remains within reach of the Serbian authorities. He can and must be arrested immediately and I expect all Serbian authorities to work much more intensely towards that objective," the chief prosecutor told reporters.
The EU decides next week on whether to continue the SAA talks with Belgrade or freeze them as punishment for not arresting Mladic.
"Serbia knows that negotiations may be suspended or may never be concluded if Belgrade fails to cooperate fully with the ICTY," del Ponte said.
The conditionality imposed by the European Union in the context of the negotiations on a stabilization and association agreement is of key importance, she said, urging the EU to set the final deadline by which the country must meet the set conditions and threaten to impose sanctions on Belgrade over Mladic.
"I need now a stronger support of the European Union to have Mladic in The Hague very, very soon. Clear deadlines associated with clear sanctions will produce early results," she said.
Mladic was indicted along with his political boss Radovan Karadzic in 1995 for genocide during a 43-month siege of Sarajevo, which claimed 12,000 lives, war crimes in eastern and north-western Bosnia and for orchestrating the 1995 massacre of 8,000 unarmed Muslims at Srebrenica.
"Mladic and Karadzic must be brought to justice in The Hague so that the genocide of 8,000 Muslims in Srebrenica is not left unpunished," del Ponte said.