A Cambodian psychiatrist treating victims of the Khmer Rouge and a French environmentalist cleaning up Indonesian rivers were among the winners Wednesday of the 2022 Ramon Magsaysay Award -- considered Asia's Nobel Prize.

The annual award, established in 1957 and named after a Philippines president who died in a plane crash, honours those who have performed "selfless service to the peoples of Asia".

The foundation that runs the award announced four winners in an online announcement.

Among them was Sotheara Chhim, 54, a psychiatrist and survivor of the murderous, ultra-Maoist Khmer Rouge regime that killed nearly a quarter of Cambodia's population through starvation, overwork and mass executions in the 1970s.

He was cited for devoting his life to helping people who suffered under the Khmer Rouge, with a focus on treating "baksbat" -- "broken courage" -- a syndrome seen in Cambodia that is similar to post-traumatic stress disorder.

The Magsaysay Award organisers praised "his calm courage in surmounting deep trauma to become his people's healer".

Sotheara Chhim also testified as an expert witness before a United Nations-backed tribunal trying senior Khmer Rouge leaders.

"I'm... traumatised myself as a victim under the Khmer Rouge, but working to help survivors of the Khmer Rouge helped me heal myself too," he said in a 2017 interview.

French environmental activist and filmmaker Gary Bencheghib, 27, was given the award for his efforts to clean up Indonesia's polluted waterways.

Bencheghib and his brother have built kayaks made of plastic bottles and bamboo to pick up trash in the Citarum river, one of the most polluted rivers in the world.

Filipino doctor Bernadette Madrid, 64, got the award for setting up child protection centres across the Philippines to help domestic abuse victims.

Japanese opthalmologist Tadashi Hattori, 58, was honoured for providing free eye surgeries in Vietnam, where such specialists and facilities are limited.

His generosity, the Magsaysay Award foundation said, was "the embodiment of individual social responsibility".

An in-person ceremony honouring the winners will be held in Manila in November.