Even a small bar of chocolate can trigger a big row in the Balkans.

The president of Croatia has issued a public apology for handing out packages of sweets that included Serbia-made chocolate to children in Dubrovnik, the ancient Adriatic Sea resort attacked and damaged by the Serb-led Yugoslav army in 1991.

President Kolinda Grabar Kitarovic's made the apology under pressure from Croatian nationalists.

But Serb politicians responded with anger to her gesture, calling for a boycott of Croatian goods in Serbia. Some even demanded cutting ties between the two neighboring countries.

Grabar Kitarovic said she was "unpleasantly surprised" to learn the packages distributed in Dubrovnik this week included the Serbian chocolate product after the children's parents complained.

"We will apologize to parents and replace the chocolate with Croatian products," Grabar Kitarovic said Wednesday. "Products that were not Croatian-made and which are sold in the Croatian market found their way into that package ... that will not happen again."

An investigation has shown the Serbian-made sweets were "planted" inside the treat packages by a Croatian company with a supermarket chain in Serbia, Croatian media reported.

Although the row triggered humorous and ironic comments on social media in both countries, Serbian officials were not amused.

Serbian President Tomislav Nikolic said the Croatian president's apology was "a clear sign that Croatia does not want good relations with Serbia" and that talk about reconciliation has "crumbled."

"The countries that are at a war do not buy products from each other," Nikolic said. "I'm unaware that Croatia has declared a war on us."

The Serb-led Yugoslav army and Croatia's pro-independence rebels fought a bloody war in the 1990s. After a peace deal was reached, relations slowly normalized between the two former Yugoslav republics. However, tensions have returned since recent elections that brought nationalists back to power in both countries.