He was speaking with Croatian President Stjepan Mesic during a brief meeting on the margins of a conference on democracy and combating terrorism.
"Spain would like the ICTY to be able to confirm that Croatia is cooperating in the search for the Hague tribunal's indictee, and that it is meeting the conditions necessary for the start of the negotiations with the EU," Zapatero was quoted by the Spanish government's media service as saying.
The service said Spain knew Croatia was sparing no effort to meet the requests of the International Criminal Tribunal for the former Yugoslavia (ICTY).
Spain is an EU member which has always advocated enlargement and did not set any conditions for last year's admission of 10 new members. It is not making any either for the four new candidates -- Romania, Bulgaria, Croatia, and Turkey.