Serbia and Montenegro will open talks on closer ties with the European Union later this year, the first step towards joining the EU.
Serbian President Boris Tadic has courted the European Union
EU Enlargement Commissioner Olli Rehn said Serbia must push reforms and extradite wanted men to the UN war crimes tribunal for talks to succeed.
Visiting Belgrade, he told politicians there is "no shortcut" to the EU.
Serbia and Montenegro are among the last Balkan nations to begin "stabilisation" talks with the EU.
Talks on joining the stabilisation and association process, which offers generous financial aid and is seen as a precursor to EU entry, are now expected to begin in October.
Mr Rehn and Serbian Prime Minister Vojislav Kostunica agreed to begin talks to mark the fifth anniversary of the fall of Serbia's nationalist former leader Slobodan Milosevic, the Agence France-Presse news agency reports.
Mr Milosevic, who faces genocide charges at the UN tribunal at The Hague, quit as prime minister on 6 October 2000.
"We have received a ticket to a key European match and to win in this game we must carry through precise and fast reforms," Serbian President Boris Tadic said.
Mr Rehn stressed that Serbia and Montenegro must continue a fast pace of reform, most notably in the judiciary and in efforts to combat organised crime.
In recent years Mr Kostunica has repeatedly sought closer ties to the EU and has stepped up co-operation with the tribunal.
Mr Rehn said the extradition of key figures involved in Balkan conflicts of the 1990s remains essential.
Chief among those sought are former Bosnian Serb leader Radovan Karadzic and his military commander, General Ratko Mladic.
Mr Mladic is widely believed to be hiding in Serbia with the aid of elements of the army and security services.
The two men have been indicted for their alleged role in the killing of up to 8,000 Bosnian Muslims in Srebrenica, Bosnia.