The EU has opened membership talks with Croatia, after the UN chief war crimes prosecutor said Zagreb was fully co-operating with the tribunal.
Ms del Ponte met Croatian leaders on Friday
Diplomats said the talks began in the early hours of Tuesday, shortly after a similar ceremony for Turkey.
The negotiations, which were due to start in March, stalled over Croatia's failure to hand over a top suspect.
General Ante Gotovina, charged with war crimes against Serb civilians during a 1995 offensive, remains at large.
Ms del Ponte made her report to the EU after visiting Croatia and the surrounding region last week.
It was "warmly welcomed" by EU foreign ministers, they said in a statement.
On Friday, after meetings with Croatian President Stjepan Mesic and Prime Minister Ivo Sanader, Ms del Ponte expressed disappointment at the level of Croatia's co-operation with the tribunal.
But on Monday she told the EU foreign ministers: "I can say that, for a few weeks now, Croatia has been co-operating fully with us and is doing everything it can to locate and arrest Ante Gotovina.
"Since May, the performance of the relevant services has significantly improved. There is no evidence that information has been deliberately hidden from us or from other relevant Croatian agencies."
Earlier on Monday, the EU reached a deal clearing the way for accession talks with Turkey after Croatia's ally Austria lifted its objections.