WASHINGTON (Reuters) - The United States imposed sanctions on Bosnian Serb nationalist leader Milorad Dodik on Tuesday for actively obstructing efforts to implement the 1995 Dayton Accords that ended the more than three-year war in Bosnia and Herzegovina.

Milorad Dodik, President of Republika Srpska, speaks in Pale, Bosnia and Herzegovina, September 25, 2016. REUTERS/Dado Ruvic

Dodik is the president of Republika Srpska, the autonomous Bosnian Serb half of the country established by the agreements.

Once praised as a democratic reformer, Dodik oversaw the holding of a referendum in September on celebrating “The Day of Republika Srpska” on January 9 in defiance of a Constitutional Court ruling banning the vote for discriminating against non-Serbs.

U.S. officials condemned the vote as breaching the rule of law and an attempt to undermine the peace accords.

“Milorad Dodik has defied the Constitutional Court of BiH (Bosnia and Herzegovina), violated the rule of law and poses a significant risk of obstructing the implementation of the Dayton accords,” U.S. Ambassador Maureen Cormack said in a video message on the U.S. Embassy Twitter account.

The United States and the European Union opposed the referendum. The vote was praised by Russia, with which Dodik advocates closer ties.

Dodik has called for independence for Republika Srpska, questioned the legitimacy of the judiciary, threatened to hold a referendum on the status of the court system as well as withdraw Republika Srpska soldiers from the country’s unified military.

The sanctions imposed by the Office of Foreign Asset Control, part of the U.S. Department of the Treasury, enable U.S. authorities to block access by Dodik to any of his property or assets that are under U.S. jurisdiction.

The sanctions came several weeks after Dodik said he had been invited to Friday’s swearing-in of President-elect Donald Trump, but was refused a diplomatic visa by the U.S. embassy in Sarajevo.

As it turned out, the invitation was to a private ball sponsored by religious and conservative groups. He said he would seek a regular U.S. visa.

The imposition of the sanctions “have nothing to do with the visa request,” said a U.S. official, speaking on condition of anonymity, adding that the invitation to the ball was extended to Dodik by a congressman who he did not identity.

“They (the sanctions) are part and parcel of a result of Dodik’s call for secession, his defiance of the constitutional court and (his) threats to withdraw the Republika Srpska members of the armed forces from the military,” the U.S. official added.