He is wanted for war crimes carried out in the 1992-95 war in Bosnia, including his alleged role in the 1995 massacre of about 8,000 Muslim men and boys in Srebrenica.
Mladic had lived freely in Serbia under protection from the nationalists before going into hiding a few years ago.
At the end of last year, the head of a Serbian unit hunting for Mladic resigned over his team's failure to arrest the fugitive.
Rasim Ljajic had said in September that he would quit if the former Bosnian Serb army commander was not handed over to a UN war crimes tribunal by the end of 2009.
Aljosa Milenkovic, Al Jazeera's correspondent in Belgrade, said Mladic still remains a popular figure in Serbia.
"Mladic and former Bosnian Serb leader Radovan Karadzic are still regarded by opinion polls here in Serbia as important figures that shouldn't be apprehended.
"According to the latest opinions polls the majority of Serbs will not like to see Mladic arrested and delivered to the Hague.
"But when people are asked if they would trade Ratko Mladic for a better European future, then the majority of Serbs are willing and ready to do that. So Mladic at large is a big problem for Serbs and Serbia's future."