Defense lawyers for a top Khmer Rouge leader charged with genocide have filed a complaint against Cambodian Prime Minister Hun Sen, accusing him of interfering in the U.N.-backed war crimes tribunal.

Lawyers for Nuon Chea, the Number Two Khmer Rouge leader, say the prime minister and several other officials have interfered with the defendant's right to a fair trial. They say this included unlawfully blocking witnesses from testifying and trying to prevent investigations into new cases.

The government has denied interfering in the tribunal's work.

The lawsuit comes just days after the head of the United Nations' Office of Legal Affairs, Patricia O'Brien, called on Cambodian authorities to stop making statements opposing the progress of two of the court's four main cases.

On October 9, German judge Siegfried Blunk resigned from the tribunal complaining of government interference.

Nuon Chea is set to go on trial next month along with three other defendants. They face several charges, including crimes against humanity, for their part in the deaths of up to two million people during the period of Khmer Rouge rule in the late 1970s.

The U.N.-backed tribunal has so far convicted one former Khmer Rouge prison warden and is trying the top four surviving Khmer Rouge leaders in a case that is expected to last for years.