Gen Mladic has been indicted over the 1995 Srebrenica massacre
Carla del Ponte told the BBC that Gen Mladic was within "immediate reach" of the Belgrade authorities.
Her comments came after a flurry of reports from Serbian media suggesting that the net was closing on Gen Mladic.
The former Bosnian Serb commander is wanted in The Hague to face war crimes charges including genocide.
"The authorities in Belgrade can arrest and transfer Mladic... I'm sure they can do it... it is my personal opinion, I think they can do it today, if they want," Ms del Ponte told the BBC.
She said she believed the next few days could be crucial in determining his fate.
On Wednesday she said there was no indication yet that Gen Mladic's surrender was being negotiated.
EU warns Serbia
Gen Mladic has been indicted over the siege of Sarajevo, which claimed at least 10,000 lives, and the 1995 Srebrenica massacre, in which at least 7,500 Muslim men and boys were killed. It was the worst atrocity in Europe since World War II.
The EU's Enlargement Commissioner, Olli Rehn, warned Serbia on Thursday that its talks on closer ties with the EU would be disrupted if it failed to co-operate fully with the United Nations war crimes tribunal in The Hague.
He told a committee of the European parliament that co-operation meant delivering Gen Mladic to The Hague.
"Negotiations should be suspended if the commission judges that Serbia and Montenegro at any time does not satisfactorily address this," he said.
Mr Rehn is due to present a report early next week to EU foreign ministers on whether Serbia is co-operating with the tribunal.
Mr Rehn told the BBC that the next round of talks on a new trade and aid agreement with Serbia on 5 April could be suspended unless Gen Mladic was handed over.
Ms del Ponte told reporters that Belgrade had assured her on Tuesday that "Mladic remains at large".
However, she said there was "no doubt" he was in Serbia, and had been since 1998.
The fugitive was Bosnian Serb leader Radovan Karadzic's army chief during the Bosnian war from 1992 to 1995.
Gen Mladic lived openly in Belgrade for some time after the conflict, but he disappeared from view when former Yugoslav President Slobodan Milosevic was arrested in 2001.Some of the suspect's former aides have surrendered to the war crimes tribunal to face charges of ethnic cleansing. BBC