The United Nations says Cambodia is refusing to permit a Swiss investigating judge to take his place on the tribunal trying suspected Khmer Rouge war crimes.

A spokesman for U.N. Secretary-General Ban Ki-moon said Friday the U.N. has been formally notified of Cambodia's decision not to appoint Swiss magistrate Laurent Kasper-Ansermet to the court.

The spokesman described the decision as a “matter of serious concern,” saying it breaches the terms of the 2003 agreement between Cambodia and the United Nations under which the tribunal was established.

Kasper-Ansermet was to have filled a vacancy created by the departure of German judge Siegfried Blunt — who resigned late last year complaining of interference by the government of Prime Minister Hun Sen.

The tribunal has convicted a notorious Khmer Rouge prison warden and is currently hearing a second case involving three top former Khmer Rouge leaders. But as long as the post remains vacant, the court cannot bring anyone else to trial.

Shortly after his arrival in Cambodia, Kasper-Ansermet became embroiled in an embarrassing public argument with his Cambodian co-investigating judge, You Bunleng. Kasper-Ansermet issued a press statement charging that You Bunleng was blocking him from releasing important information about two additional cases.

The Cambodian replied that Kasper-Ansermet was not yet legally accredited to the court.