The United Nations voiced concern Wednesday over Cambodia\'s delay in appointing a foreign judge to the Khmer Rouge tribunal, paralysing probes into two cases strongly opposed by the government. Swiss judge Laurent Kasper-Ansermet arrived in Phnom Penh last month as the UN\'s choice to replace a German judge who abruptly quit in October over government opposition to further prosecutions linked to the 1975-1979 regime. \"The United Nations has since made every effort to secure the appointment of the judge,\" UN chief Ban Ki-moon\'s spokesman Martin Nesirky told AFP, adding Cambodia had \"an obligation\" to appoint the reserve choice in the case of a vacancy. But the government body charged with rubber-stamping the nomination has failed to meet in recent weeks, leaving the Swiss judge in a legal impasse, in the latest setback to the court. \"The United Nations is concerned that, more than three months after the resignation of the international co-investigating judge, the Supreme Council of the Magistracy has not appointed the reserve international co-investigating judge to replace him,\" Nesirky said. The Cambodian justice ministry confirmed that it had received a letter from the UN requesting the council to discuss the matter. \"We don\'t know when the meeting will take place,\" cabinet chief Sam Pracheameanith told AFP, declining to elaborate. Observers from the US-based Open Society Justice Initiative said in a statement that Cambodia was stalling, \"effectively leaving the judicial investigations in a state of limbo\". Kasper-Ansermet\'s Cambodian counterpart You Bunleng on Monday publicly refused to work with the Swiss, who he said was not legally accredited. Kasper-Ansermet, meanwhile, accused You Bunleng of blocking \"important\" information about the two new cases involving five ex-Khmer Rouge members accused of crimes against humanity. The tribunal has so far completed just one trial -- jailing Kaing Guek Eav, a former Khmer Rouge prison chief, for 30 years in July 2010 for overseeing the deaths of 15,000 people under the brutal regime. A second trial involving the regime\'s four most senior surviving leaders is ongoing but the landmark proceedings risk being overshadowed by the controversy over the possible new cases.