Japan has agreed to donate more than $21m to support the UN-backed trials of former Khmer Rouge leaders in Cambodia.
Khmer Rouge victims have had a long wait for justice
It is the largest donation received so far to fund the trials, which were agreed after five years of talks between the government and the UN.
More than a million Cambodians died from starvation, disease or execution under the Khmer Rouge's 1975-79 rule.
But the UN has repeatedly warned that the culprits could not be tried unless more than $55m was found in funding.
Fumiaki Takahashi, Japan's ambassador to Cambodia, said his country's parliament had approved $18.5m for the tribunal, in addition to $3m already approved under its 2004 budget.
He also called on other countries to provide money for the trials.
KHMER ROUGE TRIBUNAL
Will try cases of genocide and crimes against humanity
Five judges (three Cambodian) sit in trial court
Cases decided by majority
Maximum penalty is life imprisonment
"I hope that other donors take this matter seriously, not just to show interest but also to act to contribute in concrete terms," Mr Takahashi told the Associated Press.
Australia, Britain and France have contributed about $7m between them.
Cambodia itself has pledged $13.3m, although some of this is expected to be met by donors.
About 10 people are expected to be face trial for abuses during the Khmer Rouge regime.
But only two senior figures - former torture centre director Kaing Khek Iev and army chief Ta Mok - are already in detention, awaiting trial.
The Khmer Rouge leader Pol Pot died in 1998, but many of his top henchmen are still living freely in Cambodia.
Last month Cambodian Prime Minister Hun Sen urged the tribunal to speed up preparations to try surviving Khmer Rouge leaders, or risk key defendants dying before justice could be done.