Mr Brammertz (left) said Mladic and Hadzic were "key objectives"
The UN's Balkan war crimes prosecutor says he is hopeful that Serbia will follow the arrest of Radovan Karadzic by capturing other fugitives.
Serge Brammertz said the seizure of Bosnian Serb army chief Ratko Mladic and Croatian Serb leader Goran Hadzic were his "key objectives".
The European Union has said Serbia's bid for membership depends on its handing over war crimes suspects.
Mr Karadzic's capture was warmly welcomed internationally.
Mr Brammertz said arresting the former Bosnian Serb political leader was an "important achievement".
"The commitment and the expertise of the team allow me to express careful optimism that the search for the remaining fugitives will be successful," Mr Brammertz said.
Mr Brammertz arrived on Wednesday for a two-day visit, and met Serbian officials in charge of hunting down war crimes suspects.
It is the first time that the chief prosecutor of the International Criminal Tribunal for the former Yugoslavia (ICTY) has visited Serbia since the arrest of Mr Karadzic.
He is expected to present a report on the extent of Serbia's co-operation with the tribunal to the UN Security Council at the end of the year.
However, EU officials are due to meet next week to decide whether to implement a pre-entry deal with Belgrade, which was passed by the Serbian parliament this week.
Serbian government minister Rasim Ljajic said the country was well aware of its co-operation obligations.
"We expect the EU council of ministers to adequately value what has been done in co-operation with the Hague Tribunal," Mr Ljajic said.
"We will do all we can to locate, arrest and hand over to The Hague tribunal the remaining fugitives."
The Serbian official in charge of the hunt for the suspects, Vladimir Vukcevic, is quoted as telling the AFP news agency that efforts have been redoubled, with no fewer than 10,000 officials involved.
The BBC's Helen Fawkes, in Belgrade, says that Serbia will now be hoping for positive signals from the prosecutor on its co-operation with the court.
Mr Hadzic and Gen Mladic are believed to be hiding somewhere in Serbia
While the extradition of Mr Karadzic has been praised by both the ICTY and the EU, it is still not enough, she says.
Serbia has to arrest both Gen Mladic and Mr Hadzic if it is to move closer to Europe. It is widely suspected that the men are hiding somewhere within the country.
Gen Mladic, who commanded the Bosnian Serb army, was indicted by the ICTY in 1995 on 15 counts of of genocide and other crimes against humanity in Bosnia-Hercegovina - including the massacre of at least 7,500 Muslim men and boys from Srebrenica in 1995.
Mr Hadzic was a central figure in the self-proclaimed Serb republic of Krajina from 1992 to 1993.
In 2004, he was indicted by the ICTY on 14 counts of war crimes and crimes against humanity for his involvement in atrocities committed by Serb troops in Croatia during the 1991-95 civil war.
Belgrade has been criticised for years for its failure to capture some of the most wanted war crimes suspects.