The EU froze negotiations on a stabilization and association agreement with Serbia last May, demanding that Belgrade fully cooperate with the International Criminal Tribunal on the former Yugoslavia.
But several EU members have recently urged the bloc not to isolate Serbia.
Former Bosnian-Serb military commander Mladic, one of The Hague-based tribunal's most-wanted fugitives, is accused of genocide and war crimes for atrocities that include the Srebrenica massacre of some 8,000 men and boys.
Del Ponte confirmed on January 30 she is to retire from her post in September, saying, "It's time for me to go back to a normal life."
(AP, Reuters, dpa)