Soldiers allegedly raped and sexually abused women during the crackdown
The US says it has evidence that South African mercenaries are training supporters of Guinea's military junta.
Deputy Assistant Secretary of State for Africa William Fitzgerald told the BBC mercenaries had been seen at a camp south of the Guinean capital, Conakry.
South African officials have already promised to investigate the reports which have been in the French media.
Guinea's junta has been fiercely criticised for a deadly crackdown on opposition supporters in September.
Human rights groups say more than 150 people were killed when troops fired on an anti-government protest and many women were systematically raped.
Guinean officials say 57 people died and that most were trampled to death.
The demonstration was called in protest at rumours that Capt Moussa Dadis Camara, who seized power last December, planned to run for president next year despite a promise not to do so.
Mr Fitzgerald said he agreed with reports that Capt Camara was trying to shore up his position by recruiting mercenaries.
"Some of us have seen the South Africans who are up at a camp called Forecariah south of the city - so yes that is a cause for concern," he said.
"I believe that they are military trainers."
South Africa's director general of International Relations Ayanda Ntsaluba earlier said the mercenary allegations were being taken seriously.
"The allegation is that there is a group of South Africans, mercenaries who are training militia largely recruited on an ethnic basis, supporters of the current military youth," he said.
He indicated that the information pointed to companies operating through Dubai.
It also seemed to point to a "strong South African connection", he said, but other South African officials were being cautious about the leads.
South Africa has strict laws forbidding mercenary activities.
Last month, the UN created a tribunal to investigate the killings in Guinea's capital Conakry on 28 September.
The US and the African Union have imposed sanctions against Capt Camara and 41 members of his junta.
Former colonial power France and West African countries have imposed an arms embargo, while the EU has called for Capt Camara to be put on trial for crimes against humanity.