Serbian security forces raided five locations in Belgrade on Thursday as part of the search for Bosnian Serb war crimes fugitive Ratko Mladic, including the home of his son, police said.

Outfitted with all-black uniforms, including balaclavas, and heavy automatic weapons, dozens of elite police blocked the street with jeeps before entering son Darko Mladic's house in the leafy Belgrade suburb of Banovo Brdo.

"We are searching for Mladic," a police source told AFP. The raids had got underway at 9:30 am (0830 GMT) in Belgrade at the request of the investigating magistrate of the Serbian war crimes tribunal, he added.

Police chief Milorad Veljovic said the operation was part of measures to track down Mladic and Goran Hadzic, the only other fugitive still wanted by The Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY).

"We have to fulfill our obligations towards The Hague tribunal," Veljovic told RTS state television.

However Rasim Ljajic, the minister in charge of Serbian cooperation with the ICTY, said the raids were aimed at finding evidence to locate Mladic, the wartime Bosnian Serb general wanted for genocide.

"Let me be clear, we are not looking for Ratko Mladic. We are looking for traces, proof, documents and any kind of evidence that could lead to Ratko Mladic," Ljajic told B92 television.

"These actions will be conducted during the day, and in the coming days, and they will not stop until the two remaining indictees are found and delivered to The Hague-based tribunal," he said.

The raid, which police said also included three private flats in various parts of Belgrade as well as the suburban office of Darko Mladic's wife, was over around 5 pm (1600 GMT).

State-controlled Tanjug news agency quoted a family friend who witnessed the search as saying "the police took every single peace of clothing, small paper and thing they considered could be helpful in Mladic's arrest."

The Serbian operation came only a day after NATO peacekeepers in Bosnia raided the home of Radovan Karadzic's wife, over suspected links with Mladic.

"We have reason to believe that Karadzic's and Mladic's network overlap... and we wanted to talk to members of (the) Karadzic family about possible communication with Mladic's support network," a NATO spokesman said.

"It was a very worthwhile operation," he added without elaborating on the operation in Karadzic's wartime stronghold of Pale, a village near Bosnia's capital Sarajevo.

Karadzic, the political leader of Serbs during Bosnia's 1992-1995 war, was captured in Belgrade in July. He is awaiting trial in The Hague.

Mladic, 66, is wanted for genocide, war crimes and crimes against humanity by the International Criminal Tribunal for the former Yugoslavia (ICTY) for his role in the Bosnian conflict.

Mladic and Karadzic are linked to some of the worst atrocities in Europe since World War II, including the July 1995 Srebrenica massacre of 8,000 Bosnian Muslims and the siege of Sarajevo that claimed more than 10,000 lives.

Cooperation with the ICTY -- including the capture of war crimes indictees -- is a precondition for Serbia's hopes of closer relations with, and membership in, the European Union.