By Prak Chan Thul

PHNOM PENH (Reuters) - The first member of Cambodia's notorious Khmer Rouge regime jailed for the 1970s "Killing Fields" atrocities admitted on Thursday brutally murdering four unidentified Westerners and burning their bodies with piles of tyres.

Kaing Guek Eav, alias "Duch", is testifying at an international tribunal's long-running second case against the deputies of late Khmer Rouge leader Pol Pot, whose four-year reign of terror in pursuit of a peasant utopia killed at least 1.8 million Cambodians.

Duch said "Brother Number Two" Nuon Chea had personally instructed him to execute four Westerners, including two Americans, at a school that was turned into a torture center, where more than 14,000 people were killed.

He said the foreigners were killed because they had trespassed into Cambodian waters. The identity of the foreigners remains unknown.

"They were interrogated and smashed per instruction," Duch told the court.

"They had to be burnt to ashes so there is no evidence that foreigners were smashed by us."

Most of the Khmer Rouge victims died of starvation, torture, exhaustion or disease in labor camps, or were bludgeoned to death during mass executions carried out across the country.

The majority of Cambodians alive now were born after the bloody era and are enjoying a peace and growth and embracing the capitalism the Khmer Rouge had deplored.

Nuon Chea and former head of state Khieu Samphan are on trial at the U.N.-backed court for war crimes and genocide. Now in their 80s and in declining health, they were sentenced to life imprisonment in 2014 for crimes against humanity.

Their complex case was divided into two to ensure justice was delivered while they were still alive. Two of their co-defendants, Ieng Sary and Ieng Thirith, are dead.

Pol Pot died in 1998.

Though other aging cadres have been indicted, there is little optimism a decade-old tribunal fraught with delays, political interference and funding problems can bring justice and closure to Cambodia.

Duch, 73, was jailed for life in 2010 for crimes against humanity. Earlier this week, he reiterated he was only acting on instructions from the Khmer Rouge's upper echelons and from Nuon Chea to execute prisoners.

"No form of punishment on earth would be fair for what they did to the four foreigners and millions of Cambodians and their family members," said Youk Chhang, director of the Documentation Center of Cambodia.

The Documentation Center, which has conducted research into the killings by the Khmer Rouge, lists 79 foreigners among those killed at the Tuol Sleng torture center, most of them Vietnamese and Thai but including four Americans, three French, two Australians, one Briton and one New Zealander.

Some of them were known to have strayed into Cambodian waters while sailing through the Gulf of Thailand.

(Editing by Martin Petty and Robert Birsel)