SARAJEVO, Bosnia and Herzegovina — A pro-Russia Serbian nationalist claimed victory in the race for the Serbian seat on Bosnia’s three-member presidency on Sunday but official results will not be announced until Monday morning.

A victory for Milorad Dodik, a Bosnian Serb strongman and a close ally of Russia’s president Vladimir V. Putin, could exacerbate ethnic rivalries in the Balkan nation and stall the country’s bid for membership in the European Union.

The president of the country’s election commission, Branko Petric, confirmed that with 44 percent of the vote counted, Mr. Dodik was leading in the race for the Serbian seat in the country’s tripartite presidency.

“It’s a clear cut victory,” Mr. Dodik said in Banja Luka, the capital of the Serbian autonomous region of Bosnia. “I don’t care who the other two representatives in the presidency are. I am going there, to this presidency, to work above all and only for the interests of Serbs.”