One of four former Khmer Rouge leaders on trial for war crimes in Cambodia is arguing he should not be tried by a United Nations-backed tribunal for genocide because he has already been convicted of the crime and pardoned.

Lawyers for former Khmer Rouge Foreign Minister Ieng Sary pressed their arguments Tuesday, on the second day of a long-awaited war crimes trial for the four most senior survivors of the hard line communist movement, which decimated the country in the late 1970s.

The lawyers argue their 85-year-old client was convicted of genocide in 1979 by a Vietnamese-backed tribunal, after Khmer Rouge leader Pol Pot was ousted from power. Cambodia's king later pardoned Ieng Sary, granting him and his followers an amnesty after he led a mass defection of Khmer Rouge guerrillas to the government. Lawyer Michael Karnavas, speaking Tuesday, said his client has abided by all conditions of that amnesty, and said it helped bring peace to the war-ravaged country.

On trial with Ieng Sary are 79-year-old Khieu Samphan, the nominal Khmer Rouge head of state; 84-year-old Nuon Chea, described as the regime's chief ideologue; and Ieng Sary's 79-year-old wife, Ieng Thirith, who was minister of social affairs. Khmer Rouge leader Pol Pot died in 1998.

The charges against the defendants include religious persecution, torture and genocide linked to the deaths of as many as 2 million people between 1975 and 1979.

The trial, which analysts say could take years, is expected to deal with preliminary arguments in coming weeks. The tribunal is expected to begin hearing testimony from witnesses in August or September.

The trial is the showcase event for the U.N.-backed tribunal, which was created to demonstrate impartial justice and foster national healing.