UN war crimes prosecutor Carla del Ponte has accused Croatia of failing to arrest a top war crimes suspect.
Gen Ante Gotovina is accused of war crimes against ethnic Serbs
Fugitive general Ante Gotovina is the Hague war crimes tribunal's third most wanted person.
The Croatian Foreign Ministry replied in a letter to the EU Presidency it will do its "utmost" to co-operate.
The EU has already said that if Croatia fails to hand over Gen Gotovina by 17 March, accession talks due to go ahead on that date will not take place.
Ms Del Ponte was adamant that not enough has been done by the Croatian government.
"We have information coming from a lot of directions that Gotovina is in Croatia," Ms del Ponte told a news conference in Warsaw on Thursday.
"I know, I'm sure that the Croatian government was trying to persuade Gotovina [to surrender] but they were not ready to arrest him for many reasons," she added.
"I am asking now the Croatian government to take further steps to do that."
Gen Gotovina is accused of ordering the killing of 150 ethnic Serbs and the expulsion of 150,000 in 1995.
But he is also regarded as a hero by many Croats, who would strongly oppose his arrest.
The war crimes he allegedly committed against Serbs took place during and after a Croatian army offensive against rebel Serbs in the Krajina region in 1995.
The Croatian government has vowed to do whatever it can "to pursue the resolution of Gotovina's case", the letter from the Foreign Ministry said.
It added that the arrest and handover of the former general "remains an unavoidable obligation".