Croatia's conservative opposition has won the Balkan country's first parliamentary election since joining the European Union in 2013, but without enough votes to rule alone.

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The state electoral commission said on Monday that with 99 percent of the votes counted, the conservatives, led by former intelligence chief Tomislav Karamarko, won 59 seats in the 151-seat parliament.

The ruling Social Democrats, led by incumbent Prime Minister Zoran Milanovic, got 56 seats.

The result means both blocs have failed to win an outright majority and the forming of the new government will depend on several small parties that entered parliament.

The kingmaker will be the third-placed party, Most, or "Bridge", with 19 seats.

The election on Sunday was held amid a huge refugee surge through Croatia and deep economic woes facing the country.

The HDZ, now led by Karamarko, was ousted from power four years ago amid a series of unprecedented scandals involving its former leader, Ivo Sanader.

But the centre-left government, at the helm since then, has disappointed voters by failing to reform the public sector and boost the business climate, although Croatia saw a slight return to economic growth this year after six years of recession.

Source: Agencies