Croatian Prime Minister Andrej Plenkovic on Monday criticised the mayor of tourist resort Dubrovnik for publicly using a slogan of the country's World War II pro-Nazi regime.

Dubrovnik mayor Mato Frankovic, at the opening of a memorial park to veterans of Croatia's 1990s war of independence, ended his speech with the slogan "For Homeland -- Ready" ("Za Dom -- Spremni" in Croatian) used by the pro-Nazi Ustasha regime.

The Sunday event took place near the southern town of Imotski.

After the use of the slogan was strongly criticised by the opposition, Frankovic argued on Facebook he did not use it in the "context of WWII" but rather of the 1991-1995 war to break away from Serb-dominated Yugoslavia, when it was used by Croatian paramilitary group HOS.

Conservative Prime Minister Andrej Plenkovic on Monday labelled the mayor's move unacceptable.

The prime minister told reporters it was neither his HDZ party nor the government's policy to end speeches with the slogan.

"It is politically unacceptable for us in every sense."

He added that the institutions would decide on "possible consequences" for the mayor, who also comes from the HDZ party.

The Ustasha, in line with their racial laws, persecuted and killed hundreds of thousands of ethnic Serbs, Jews, Roma and anti-fascist Croatians.

In recent years the European Union country has seen a growing tolerance for its pro-Nazi past and critics accuse the authorities of failing to sanction the use of the Ustasha symbols.

Although the Ustasha's so-called wartime Independent State of Croatia was a Nazi puppet, their modern sympathisers see the regime's leaders as the nation's founding fathers.