Bosnian Serbs are voting in a referendum banned by the country's constitutional court - risking western sanctions against their autonomous region and criminal charges against their leaders.

Sunday's vote is asking whether to keep January 9 as a holiday in Republika Srpska - commemorating the day in 1992 that Bosnian Serbs declared the creation of their own state and ignited the devastating 1992-95 war.

The vote comes despite the top court's ruling that the date discriminates against Muslim Bosniaks and Catholic Croats in Bosnia, who were persecuted and expelled by the Serbs.

The west has warned against holding the illegal referendum, but the Bosnian Serbs are backed by Russia.

Western officials say they might consider halting projects in the mini-state or impose travel bans on leaders and freeze their assets.