SARAJEVO, Bosnia-Herzegovina (AP) — Bosnia's Serb mini-state is holding a referendum this weekend that has turned into a proxy political battle between the West and Russia, stoking ethnic tensions and triggering fears of new clashes more than 20 years after the end of the Balkans War.

Sunday's vote asks residents of Republika Srpska whether to maintain a national holiday on Jan. 9, despite a ruling of Bosnia's constitutional court that the date discriminates against non-Serbs.

On January 9, 1992 — a Serb Christian Orthodox religious holiday — the Bosnian Serbs declared the creation of their own state within Bosnia, fueling a war that resulted in at least 100,000 people dead and millions homeless. During the worst carnage in Europe since World War II, the Serbs — helped by neighboring Serbia — expelled Bosniak Muslims and Catholic Croats from the territory they occupied, with the aim of making it part of Serbia.

A U.S.-brokered peace deal signed in 1995 created Republika Srpska, an autonomous region within Bosnia. For Bosniaks and Croats — whose federation represents the other half of Bosnia — Jan. 9 still symbolizes their expulsion and a sign that the Bosnian Serb-controlled territory is still meant just for Serbs.

Bosnia's constitutional court has banned the referendum, but the Serbs said they will hold it anyway.

The vote sparked the most heated exchange between Bosniaks and Serbs since the war in the 1990s. Serbian officials said they were ready to defend Republika Srpska if it was attacked, and ordered the Serbian army to be more vigilant.

Apart from challenging the country's rule of law, Bosniaks also fear this referendum is a test for a more serious one in 2018 — on independence — which would not go down peacefully, officials in Sarajevo warned.

"Nobody is more ready to defend this country all the way to the end," said Bosniak leader Bakir Izetbegovic. "Nobody should ... force people who love this country to prove it again," he added.

The referendum also reflects the wider tensions between Western nations — which are supporting the Bosniaks and Croats — and Russia, which is backing the Serbs.

The U.S. embassy in Sarajevo threatened unspecified "consequences" if the referendum is not canceled, while the Russian ambassador openly supported the referendum, saying it is an act of democracy. Russia is a traditional backer of Orthodox Slavic Serbs.

"Russia's economy has been hurt badly by Western sanctions imposed because of Ukraine," said Balkans expert and author Tim Judah. "If Russia can cause the West problems in return, which it seems determined to do in Bosnia now, then so be it. Never mind if this tips Bosnia back into conflict, never mind that Russia has nothing to offer the Balkans, this is simply a good way to cause problems to the West."

Bosnian Serb leader Milorad Dodik, who initiated the referendum and went to Moscow to meet President Vladimir Putin this week, called Bosniak reactions "hysteria."

"This is not a referendum about secession as many want to portray it," he said. "It is not even a beginning of such a process."

International officials overseeing Bosnian peace accords were outraged by Dodik's defiance and threats of new clashes.

"In the past 20 years we have not heard such language," said the High Representative for Bosnia, Austrian diplomat Valentin Inzko.

The Peace Implementation Council, an international body overseeing Bosnia, said there will be no redrawing of borders and called on everybody "to refrain from reactive measures and divisive rhetoric." It also urged the Bosnian Serbs to cancel the vote, but Council member Russia distanced itself from the statement.

The referendum is "both a test for state institutions as well as an attack on them," Inzko told the AP in an interview, noting also that Bosnia's criminal law foresees jail terms from six months to five years for those who disobey the constitutional court.

The sanctions could also include travel bans, asset freezes and the halting of international projects in the country, Inzko said. "All options are open," he said.

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AP Writer Dusan Stojanovic contributed from Belgrade, Serbia.