The European Union has warned Croatia that its failure to arrest a key war crimes suspect may delay entry talks.
Gen Ante Gotovina is accused of war crimes against ethnic Serbs
EU Enlargement Commissioner Olli Rehn said he was prepared to suspend talks due to start in March if General Ante Gotovina was not arrested.
He said information suggested the fugitive was within the Croatian government's reach.
The general is accused of ordering the killing of more than 100 ethnic Serbs and expelling 150,000 in 1995.
Last year, EU leaders agreed to start membership talks with Croatia, but only if it handed over General Gotovina.
Mr Rehn told a press conference in Brussels he had reliable information from the international war crimes court and other sources that General Gotovina was either in Croatia or in the Croat part of Bosnia.
He said the Croatian government had to do more to ensure his transfer to the International Criminal Court for the former Yugoslavia (ICTY) in The Hague.
"I am convinced that if the Croatian government had the political will to locate and transfer General Gotovina, it could do so. Let me also say very clearly that, without full co-operation with the ICTY, the negotiations cannot start."
Croatian Justice Minister Vesna Skare-Ozbolt insisted that "we are doing really everything to find where he is".
The BBC's European regional correspondent, Oana Lungescu, says Mr Rehn appears unwilling to make any political concessions. There is no shortcut to Europe, he warned the Croatian government, just the regular road, which means the respect of the rule of law.
Romania and Bulgaria were invited to sign entry treaties in April, ahead of their planned entry in January 2007.
But they have been told entry could still be delayed by one year if they failed to make further progress on judicial reform, fighting corruption, strengthening border controls and cutting state aid to industry.