The Khmer Rouge genocide tribunal was today facing fresh turmoil after it was announced that one of the three men standing trial for war crimes had died in hospital.

The death of Ieng Sary, 87, who served as foreign minister of the Khmer Rouge regime, underscored the concerns of many victims and families of those who died who are increasingly anxious about how slowly the trial is proceeding.

“As a victim of the Khmer Rouge, I am very disappointed that Ieng Sary escaped justice, escaped the trial,” Ou Virak, whose father was killed by the regime and who now heads the Cambodian Human Rights Centre, told The Independent from Phnom Penh.

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He added: “I am also disappointed by the pace of the trial. This is is exactly what we have been saying. There is no time to waste.”

The death of Ieng Sary means there are now only two former senior members of the Maoist-inspired regime on trial – Nuon Chea, also known as Brother Number Two and who was considered the right-hand man of Pol Pot and the regime’s ideologist, and the former president, Khieu Samphan.

As it is, a team of international experts is next week due to check the medical condition of Nuon Chea, who has been in and out of hospital for years, to determine whether or not he is well enough to continue being tried. Ieng Sary was also to have been examined by those doctors.

In 2011, the court decided that Ieng Thirith, the regime's “First Lady” and the only female leader to be charged, was unfit to continue with the proceedings after she was found to be suffering form dementia.

Lars Olsen, a spokesman for the so-called the Extraordinary Chambers in the Court of Cambodia, said Ieng Sary had died in the Khmer Soviet Friendship Hospital in Phnom Penh where he had been admitted on March 4. “He has been suffering from intestinal problems,” said Mr Larsen. “He has been hospitalised several times during the last year.”

The UN-backed tribunal has been rocked by difficulties since it was established more than a decade ago. Perhaps the most significant challenge was political interference from the government of Hun Sen, Cambodia’s prime minister and himself a former officer with the Khmer Rouge.

His interference meant an effective block on expanding the number of suspects to be tried. Experts had suggested there was sufficient evidence to bring several other former Khmer Rouge leaders before the tribunal – a preliminary examination of documents and witness statements known as cases three and four - but the moves were opposed by Cambodian lawyers in the court, apparently under the influence of Hun Sen. The country’s foreign minister openly stated Hun Sen would not allow “case three” to proceed.

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In October 2011, one of the investigating judges, Siegfried Blunk, quit his post, claiming the government of Cambodia was trying to interfere with the proceedings of the court.

The unwillingness of the Cambodian authorities to engage with the tribunal has also been exposed in other ways. In recent weeks, the trial’s proceedings ground to a halt after Cambodian staff went on strike, complaining that they had not been paid for months. The Cambodian government is responsible for paying the salaries of Cambodian staff.

Activists insist that however painful and difficult the tribunal may be, it is an essential step if Cambodia’s wish to move forward from this dark episode in its past. Anywhere up to 1.7m Cambodian civilians were murdered or else starved to death during the rule of Khmer Rouge, between April 1975 and January 1979, when they were forced from the cities and made to work on agricultural farms.

The tribunal has also raised uncomfortable questions for many of the major powers, some of whom are funding the £100m tribunal. China openly supported the regime, while a number of countries, among them the UK, permitted the Khmer Rouge to retain Cambodia’s seat at the UN General Assembly after they were ousted from power by Vietnamese troops.

The tribunal has also drawn attention to the brutalising effect of the massive secret bombing campaign of the US in Cambodia and Laos, directed against Khmer Rouge and South Vietnamese forces and which some historians have argued created the circumstances for the Khmer Rouge to seize power.

So far, the achievements of the tribunal have been modest. Only one former leader, Guek Eav, known as Comrade Duch, who headed the notorious S-21 detention centre where up to 14,000 people were tortured and dispatched to be killed, has been convicted. He was found guilty of war crimes in 2010 and sentenced to 19 years in jail.

The Associated Press reported today that Ieng Sary studied in Paris with Pol Pot and held senior positions, including that of deputy prime minister in charge of foreign affairs. When Vietnamese forces invaded Cambodia in 1979 and drove the Khmer Rouge from Phnom Penh, both Pol Pot and Ieng Sary were sentenced to death in absentia.

They fled with Khmer Rouge loyalists to remote jungle strongholds in western Cambodia. But Ieng Sary’s 1996 defection to the government, along with thousands of fighters, dealt a death blow to the movement. Pol Pot died two years later.

Ieng Sary was the public face of the Khmer rouge and had a much higher profile than many of his senior colleagues. Yet when he was arrested in 2007, Ieng Sary refused to cooperate with the court, insisting that he had been pardoned by King Norodom Sihanouk. The UN tribunal ruled that the pardon did not cover its indictment against him.