The United States is "deeply concerned" by reports that South African "military instructors" are training militiamen backed by the ruling junta in Guinea, a United States official said Thursday.

"The United States has received reports of 40 or 50 South African military instructors training militia members in Guinea, including some under the age of 18," the State Department official said on the condition of anonymity.

Witnesses in Guinea said on Monday that South African as well as Israeli army instructors, hired by the ruling junta, are training recruits in a camp in Forecariah, 100 kilometres south of the capital Conakry.

The State Department official, who also understood that the camp is in Forecariah, said: "We are deeply concerned." He did not mention Israelis.

"We view this activity as a direct affront to the democratic aspirations of the Guinean people contrary to the efforts of the international to reach a peaceful resolution to the current crisis and calculated move by the ruling junta to maintain power through armed force and ethnic division," the official said.

He said the US government has raised its concerns with the South African authorities who "promised to investigate these reports. We applaud that."

In Pretoria on Tuesday, South Africa's foreign ministry said it was investigating the reports that people described as South African mercenaries are training pro-junta recruits in Guinea.

Witnesses said the new soldiers recruited by the junta, which seized power in Guinea on December 23 last year, are being trained in a camp formerly used to house Sierra Leone refugees outside Forecariah.

Around 40 military instructors are training soldiers "recruited on the basis of their ethnicity" as they belong to the same group as junta leader Captain Moussa Dadis Camara, witnesses said.

Observers accuse the junta, under increasing international pressure after the massacre of over 150 opposition supporters at a rally in September, of recruiting young men from Camara's home region close to Liberia and Sierra Leone.