Macedonia's Foreign Ministry voiced strong concern at a speech by Anthimos, the hardline Metropolitan Bishop of Thessaloniki, who said that "Macedonia is Greek ... and parts of it that are missing should be returned."
Greece and Macedonia are at odds over the Balkan country's name, which Athens says could imply claims on the northern Greek province of Macedonia.
A Foreign Ministry statement said it was "disturbing" to hear the bishop's "extremist speech."
Anthimos is the Church of Greece's top official in northern Greece, where sentiment runs high over the name dispute.
His comments came days after Greece and Macedonia held new U.N.-supervised talks on the name issue, which has poisoned bilateral relations since 1991, when Macedonia gained independence from the former Yugoslavia.
The Foreign Ministry said it expects Greek authorities "immediately and unequivocally to condemn such statements, which pose an indirect threat to the territorial integrity of the Republic of Macedonia."
In Athens, Greek Foreign Minister Dora Bakoyannis said Thursday that the bishop's comments "do not, of course, express the (Greek) government, the people and political parties."
"Greece does not seek border changes," she said.