Serbia reacted angrily to a decision by Croatia to block its early steps toward European Union membership by demanding the government in Belgrade change laws on prosecuting crimes committed during the bloody split of former Yugoslavia.

Croatia refused to support the opening of talks on a so-called negotiating chapter dealing with judiciary and basic rights at a meeting in Brussels on Friday, saying its former Yugoslav federation partner must not be able to prosecute crimes committed in the other republics. The chapter, along with one on justice and freedom, was slated to be the first area of talks in Serbia’s path toward joining the 28-member bloc.

“Serbia is shocked with Croatia’s decision not to support Serbia’s EU path,” Prime Minister Aleksandar Vucic said in e-mailed statement. “Serbia acknowledges Croatia’s stance and will not let anyone in Europe, world, including the Republic of Croatia, to blackmail, humiliate or denigrate Serbia.”

Croatia’s refusal to acquiesce is a blow to Vucic, who initiated snap parliamentary elections set for April 24 to secure a new term so he can prepare the country of 7.2 million for EU membership by 2020. It also underscores still simmering tensions between the Balkan neighbors, which flared last week after a United Nations war crimes tribunal acquitted nationalist Serbian leader Vojislav Seselj on charges that included the murder and deportation of ethnic-Croats in the wars of the 1990s.

The EU formally opened entry talks with Serbia in January 2014, although since then it hasn’t begun discussions on any of the 35 areas that in which Serbia must meet the bloc’s norms.

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