European Union foreign policy chief Javier Solana said the advice of the prosecutor, Carla del Ponte, would be crucial when the bloc decides whether to open entry talks with Croatia.
Separately, Paddy Ashdown, EU Special Representative for Bosnia and Herzegovina, on Thursday said lack of cooperation with The Hague was the main obstacle preventing the republic from developing closer ties with the EU and Nato.
EU leaders have decided to start entry negotiations with Croatia on 17 March provided it is cooperating fully with the tribunal to bring fugitive war crimes indictee General Ante Gotovina to trial.
EU Enlargement Commissioner Olli Rehn said on Monday that on the basis of information available to him then, he could not recommend opening negotiations.
Del Ponte is due to give the EU an opinion in writing.
Solana says EU will defer to views
of Carla Del Ponte
She declined to say what her recommendation would be, telling reporters she was "staying out of politics", and referring the question to Solana - who made it clear he and the bloc would defer to her.
"We will listen to Madame del Ponte before taking any decision," Solana said. "The advice ... will be absolutely fundamental for the final decision."
Croatian President Stjepan Mesic and Prime Minister Ivo Sanader issued a statement on Tuesday, calling on all relevant institutions to step up efforts to find and arrest Gotovina.
Until then, Croatia had mainly urged Gotovina to give himself up voluntarily. Del Ponte welcomed the new approach.
"I was very pleased that Prime Minister Sanader told us publicly that he's prepared ... to arrest him," she said. "I hope it will be done very soon."
"The biggest number one priority that we have to achieve in Bosnia Herzegovina is cooperation with The Hague"
EU special representative for
Bosnia and Herzegovina
Ashdown met Solana later and said although last month's surrender of Savo Todovic, a Bosnian Serb charged with murder and torture of non-Serbs during the Bosnian war was a positive sign, much remained to be done.
"The biggest number one priority that we have to achieve in Bosnia Herzegovina is cooperation with The Hague," Ashdown said after talks with Solana. "This has been a roadblock on the path to the EU and NATO."
But he said del Ponte believed in Bosnia. "At least, there are signs of greater cooperation. But they're only signs," he said.
Another war crimes indictee, Serb General Vladimir Lazarevic, arrived in The Hague on Thursday after surrendering to Serb authorities to ease foreign pressure on Belgrade.
But Serbian Foreign Minister Vuk Draskovic acknowledged Lazarevic's surrender would not be enough to satisfy Brussels that Serbia and Montenegro was cooperating fully with The Hague.