Lawyers for a former Khmer Rouge leader demanded that investigators at Cambodia's war crimes court question premier Hun Sen and government officials over alleged interference.

The defence for former Khmer Rouge ideologue Nuon Chea cited a September statement by Hun Sen - who himself defected from the communist regime in 1977  - that witnesses do not have to testify to the Extraordinary Chambers in the Courts of Cambodia (ECCC).

"If Hun Sen has indeed told witnesses that they do not have to cooperate, then he has committed a criminal offence and is seriously affecting the judicial investigation," defence lawyer Michiel Pestman said.

A copy of the request also cited a government spokesman's remark in October that six senior government and legislative officials summoned by the court should not testify.

"(Hun Sen's) conduct is affecting the fair trial of our client because some of these were witnesses we asked for," Pestman said.

"Ultimately it could undermine the whole legitimacy of the court," he added.

The process has often been hit by allegations that Hun Sen's administration has attempted to interfere in the tribunal to protect former regime members who are now in government.

The troubled tribunal, which has also been hit by accusations local staff were forced to pay kickbacks for their jobs, was created in 2006 to try leading members of the regime on charges of war crimes and crimes against humanity.

As the court has sought to investigate other suspects, Hun Sen has made fiery speeches warning further prosecutions could plunge Cambodia back into civil war.

The court plans to try Nuon Chea, former Khmer Rouge head of state Khieu Samphan, foreign minister Ieng Sary and his wife, minister of social affairs Ieng Thirith sometime in 2011.

The court's first trial, of former Khmer Rouge prison chief Kaing Guek Eav, alias Duch, completed its final arguments last week.

(AFP)