"If the Commission were to give its recommendation on the basis of today's information, I could not recommend opening negotiations with Croatia," Rehn told a news conference in Brussels at which he presented a negotiating framework for membership talks with Croatia.
"I trust the Croatian government will take this message seriously," the EU official said and added: "There is no short-cut to Europe, just the regular road, which means full respect for the rule of law."
Rehn went on to say that based on the information obtained from the tribunal's chief prosecutor, Carla del Ponte, and other well-informed sources it could be assumed that General Ante Gotovina was within the reach of Croatian authorities.
According to the same sources, there is reason to assume that General Gotovina, who has been on the run since July 2001 when the ICTY unsealed his indictment, is probably in Croatia or Croat-populated areas of Bosnia-Herzegovina, Rehn said.
He added that the Croatian government was at least in indirect contact with the fugitive general and that it could have located him and transferred him to The Hague if there had been the political will to do so.
It is now up to the Croatian authorities to prove that they are fully cooperating with the Hague tribunal. Full cooperation means locating and handing Gotovina over to the tribunal, he said.
"If there is no progress on the Croatian side, we are prepared to postpone accession talks," Rehn said. No one wants that, but we must be ready for that option, too, he added.
There can be no talks without full cooperation, and it should be said that the Croatian government now has enough time to act, Rehn said.
The European Council decided at a summit last December to open entry talks with Croatia on March 17 this year provided that it fully cooperated with the Hague tribunal. It asked the European Commission to draw up a negotiating framework for accession talks with Croatia.
The European Commission is to formally accept the proposed negotiating framework on Wednesday and forward it to the Council of Ministers, which should endorse it unanimously.
According to the proposed negotiating framework, the acquis communautaire, which Croatia must adopt during the negotiating process, has been divided into 35 chapters. Negotiations will be held as part of an intergovernmental conference where decisions are taken unanimously. The pace of negotiations will depend on the fulfilment of membership criteria.
Rehn said that the fulfilment of political criteria -- human rights and the rule of law -- would be closely monitored, and that so-called safeguard clauses were put in place in order to allow for the talks to be suspended in the event of serious and persistent violations of the principles of freedom, democracy and respect for human rights and fundamental freedoms.
Rehn reiterated that Croatia might become a full member in 2009 if everything went smoothly. If every step is conducted perfectly in the negotiations and prior to them, Croatia might become a member in 2009. It is an ambitious but realistic date, he said.