An international judge at the Khmer Rouge war crimes tribunal in Phnom Penh has resumed probing a new case without support from his Cambodian counterpart, fuelling a row at the UN-backed court.

Swiss judge Laurent Kasper-Ansermet said he does not need the approval of investigating judge You Bunleng to re-open a controversial investigation into allegations of mass killings by two ex-Khmer Rouge commanders.

"It is in the public interest that the judicial investigation be resumed without further ado in order to carry out the full investigation warranted by the seriousness of the offences," Kasper-Ansermet said in a statement.

The judge has been backed by the United Nations in the face of a Cambodian decision not to endorse his appointment at the court -- a move that effectively halted work on potential new prosecutions.

The Cambodian government, which includes many former Khmer Rouge members, strongly opposes going after more suspects of the 1975-1979 regime, blamed for the deaths of up to two million people, beyond the court's current second trial.

You Bunleng, who came under fire last year for closing the probe into the court's potential third case without quizzing the suspects or visiting the crime scenes, told AFP Kasper-Ansermet's move was "inappropriate" because the Swiss was not yet legally accredited.

A German investigating judge quit in October citing political interference at the court and Phnom Penh's refusal to recognise Kasper-Ansermet as his replacement has sparked a public standoff with the UN.

The tribunal completed its first trial last week, jailing former Khmer Rouge prison chief Duch for life on appeal for overseeing the deaths of some 15,000 people.

A second trial involving three senior regime leaders is ongoing but the landmark proceedings risk being overshadowed by the current controversy.

Before Kasper-Ansermet's arrival the court's third and fourth cases were widely expected to be dropped but their fate now depends on the outcome of the legal battle between the sparring judges.

Cambodia says it objects to the judge because he used Twitter to draw attention to the controversy surrounding the new cases but observers say those concerns could be an excuse to keep stalling on the investigations.