The UN's chief war crimes prosecutor has denied reports suggesting that top war crimes suspect General Ratko Mladic has been arrested.
Gen Mladic has been indicted over the 1995 Srebrenica massacre
Carla del Ponte said "the false rumours have absolutely no basis", and there was no indication that his surrender was being negotiated.
But she said the former Bosnian Serb wartime general "can and must be arrested immediately".
The Hague tribunal has charged Gen Mladic with war crimes in Bosnia.
EU warns Serbia
European Union Enlargement Commissioner Olli Rehn is due to present a report early next week to EU foreign ministers on whether Serbia is co-operating with the tribunal.
The EU warned Serbia last month that moves towards membership could be halted if Belgrade does not hand over Gen Mladic.
But on Wednesday Mr Rehn talked of a deadline.
He told the BBC 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.
"It's in the hands of Serbia now to choose whether it wants to have a European future or a nationalist past," Mr Rehn said.
"Unless Serbia is able to co-operate now without delay, it will lead to a disruption of negotiations. The next political round is scheduled for April, which means that will certainly be impacted if there is no full co-operation."
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.
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.
"During all this time he has been and remains within reach of the Serbian authorities," Ms del Ponte said.
"I urge all Serbian authorities to work much more intensively."
The prosecutor also called for the "strong support" of the EU to get Gen Mladic to The Hague.
"Clear deadlines associated with clear sanctions will produce early results," she said.
Her statement follows a flurry of Serbian media reports that the net is closing around the top war crimes suspect, despite official denials of his capture.
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.