An estimated 1.7m people died as a result of the Khmer Rouge regime
A US-based campaign group has asked the United Nations not to approve a draft deal with Cambodia to set up an international genocide court to try leaders of the Khmer Rouge.
Human Rights Watch said the agreement needed major changes in order to meet international standards of justice.
A committee of the UN General Assembly is expected to meet later this week to consider the deal.
The proposal was put forward in March after more than five years of protracted negotiations, and suggests a "mixed tribunal" made up of both Cambodian and international judges and prosecutors.
UN negotiator Hans Corell was instrumental in the proposal
Human Rights Watch said the proposed tribunal would not be able to escape the influence of the Cambodian Government, which it said had a history of interfering and intimidating the legal system.
"With Cambodia's judiciary at the centre of the tribunal, the agreement ensures that it will be politics and not law that will dominate the tribunal's work," said Mr Jendrzejczyk.
Chief UN negotiator Hans Corell - who helped negotiate the new proposal - has admitted that the format is "cumbersome", but said that complex checks and intense media scrutiny would ensure a
Amnesty International has voiced similar concerns to those of Human Rights Watch.
Amnesty's concerns cover a wide range of issues, including inadequate victim and witness protection, and the
"endemic weakness of the Cambodian judicial system".
"The draft agreement would permit these problems to interfere with the tribunal," Amnesty spokeswoman Sarah Green told BBC News Online.
An estimated 1.7 million people died at the hands of the brutal Maoist regime in the 1970s, but no Khmer Rouge leader has yet been charged.
Some analysts fear that by blocking the deal, the UN would be depriving Cambodians of the last chance to see those responsible in the dock.
Most senior Khmer Rouge figures are now elderly, and some - including overall leader Pol Pot - have already died.