The admission came on Wednesday from Serbia-Montenegro's main defence body, the Supreme Defence Council, after a review of a military intelligence report on Mladic.
The council, made up of senior civilian and army officials from Serbia and Montenegro, concluded that while the Balkan country's military "had undeniably on occasion sheltered" the former Bosnian Serb commander until 2002 at unspecified "army compounds", he later left those premises.
From June 2002, "Mladic's hiding outside army compounds was taken over by retired officers" from Serbia and from the Bosnian Serb military, as well as some civilians, the council said in a statement without providing details or naming the officers.
The report was delivered by Svetko Kovac, the chief of military intelligence, and has prompted furious media speculation.
Some Serbian media claimed that they would name Mladic's accomplices and speculate about his whereabouts in the 10 years since his indictment by the UN war crimes tribunal at The Hague.
The Vecernje Novosti daily speculated that the report would name those who had helped Mladic evade justice, including Branislav Puhalo, his former aide who remains in the military.
Mladic is wanted by the tribunal to face charges of genocide for war crimes during the war in Bosnia between 1992 and 1995, including allegedly orchestrating Europe's worst massacre since World War II, when 8,000 Muslim men and boys from Srebrenica were killed in 1995.
Belgrade has been under intense international pressure to arrest Mladic.
Serbian authorities say that they have been unable to find Mladic, despite claims by UN prosecutors that he is hiding in Serbia under the protection of hard-liners in the military.
Carla Del Ponte (L) is keen to see
Mladic on trial in The Hague
The chief UN war crimes prosecutor, Carla Del Ponte, is expected to visit Belgrade on Monday to push again for Mladic's extradition.
The Supreme Defence Council reiterated that Mladic's present whereabouts were unknown and also demanded that "all those former military professionals who had assisted Mladic be criminally prosecuted".
The report also said the military had conducted 27 separate searches for Mladic at army premises across the country since March 2003.
Last week, authorities arrested another former Mladic aide, Jovo Djogo, a retired Bosnian Serb officer. Djogo was detained for 30 days for allegedly helping Mladic, but no other details about his case were made public.