|Kaing Guek Eav, known as Duch, is so far the only person to have been prosecuted by Cambodia's tribunal [AFP]
An international judge has resigned from the UN-backed Khmer Rouge war crimes tribunal, citing political interference by Cambodia's government after he tried to investigate more suspects.
Judge Laurent Kasper-Ansermet, the Swiss co-investigator, whose predecessor Siegfried Blunk quit for similar reasons in October, said he had been constantly undermined by his Cambodian counterpart, You Bunleng.
"The situation is completely blocked," Kasper-Ansermet told AFP.
In a statement released by the court, Kasper-Ansermet said that "You Bunleng's active opposition to investigations into cases 003 and 004 has led to a dysfunctional situation".
"Judge Laurent Kasper-Ansermet considers that the present circumstances no longer allow him to properly and freely perform his duties," the statement said.
The judge will stay in his role until May 4. It is unclear whether the UN has a replacement ready, but observers said any future judge would likely face similar difficulties.
"I'm surprised by the resignation but I'm not surprised at what has led to it," said tribunal monitor Clair Duffy from the Open Society Justice Initiative.
"The UN and donors have to address the heart of the problem: that the Cambodian government is trying to control who this court investigates and prosecutes."
Blunk quit the court, blaming government interference in two potential new cases, in which five mid-level Khmer Rouge members face a string of allegations including mass killings and forced labour during the regime's 1975-1979 reign of terror.
The United Nations named Kasper-Ansermet, the reserve judge, as Blunk's replacement.
But Cambodia refused to recognise the appointment, prompting an unprecedented row and forcing the Swiss to work without the support of his Cambodian counterpart You.
The court, set up to find justice for the deaths of up to two million people under the hardline communist Khmer Rouge regime, has for years been dogged by claims of political meddling.
The Cambodian government, which includes many former Khmer Rouge members, strongly opposes pursuing more suspects of the regime beyond the current second trial.
The tribunal has so far completed just one trial, sentencing former prison chief Kaing Guek Eav, better known as Duch, to life in jail on appeal last month for overseeing the deaths of some 15,000 people at a torture prison.
Hun Sen, the prime minister, himself a former Khmer Rouge soldier, in 2010 told UN Secretary General Ban Ki-moon that more indictments were "not allowed" and had previously said he would be happy if the court packed up and left.