Roncevic headed a Croatian delegation at a meeting of representatives of US-Adriatic Charter signatory countries with Secretary-General Jaap de Hoop Scheffer.
Earlier today, he and his counterparts from Macedonia and Albania, the other signatories to the charter, signed a memorandum on sending a joint team of physicians to Afghanistan in the autumn.
After the signing, Scheffer said the memorandum proved Croatia, Albania and Macedonia participated in the strengthening of security in other countries.
The secretary-general said the three countries' preparations to join NATO were progressing very well.
Croatia, Albania and Macedonia signed the Adriatic Charter with the US in 2003, committing themselves to cooperation and reform in order to accelerate accession to NATO.
Roncevic said Zagreb would assure NATO by the alliance's next summit in 2006 or 2007 that it was cooperating fully with the Hague war crimes tribunal, thus meeting the political condition to be invited to join.