South Africa's government says it has received distress calls from 17 citizens who have joined mercenary forces in the Russia-Ukraine conflict.

The men are between the ages of 20 and 39 years and are trapped in Ukraine's war-torn Donbas region.

President Cyril Ramaphosa has ordered an investigation into the circumstances that led to the recruitment of these young men into these seemingly mercenary activities, a government spokesman said. The statement did not say which side of the conflict the South Africans were fighting for.

Working as a mercenary or fighting on behalf of another government is illegal in South Africa, unless the government authorises it.

The men were lured to join the mercenary forces under the pretext of lucrative contracts, the government said.

Spokesman Vincent Magwenya added the South African government is working through diplomatic channels to secure their return.

Magwenya said 16 of the men are from KwaZulu-Natal and one is from the Eastern Cape.

President Ramaphosa and the South African government strongly condemn the exploitation of young vulnerable people by individuals working with foreign military entities, he added.