The South African government is concerned its nationals may be working as mercenaries in Nigeria in the war against the Boko Haram militant group.
Nigerian government spokesman Mike Omeri told the BBC that foreigners were just training troops in the use of new weapons.
Nigeria has recently bought helicopter gunships and tanks from South Africa and former Soviet Union countries.
But the South African government says the men may be involved in fighting.
Nigeria and its neighbours have recently recaptured several towns and villages from Boko Haram, which is allied to Islamic State (IS).
Thousands of people have been killed, mostly in north-eastern Nigeria, since it began its insurgency in 2009.