Authorities in DR Congo say they need help from the international community to control access to a mine which has produced uranium for nuclear bombs.
The country's Mining Minister Diomi Ndongala told the BBC that dangerous activities were taking place at the Shinkolobwe mine, in Katanga province.
Earlier, the UN's nuclear watchdog said it was concerned about the mine.
The government says it shut down the mine, but the BBC's Arnaud Zajtman found 6,000 illegal miners at work.
They are extracting large amounts of material containing cobalt, copper, platinum and uranium, says our correspondent.
He said that there are few safety measures and some of the material the men are handling is radioactive.
One miner told him they start digging at 0730 in the morning and dig all day with shovels, hammers and pick axes.
The uranium is allegedly sold to nearby furnaces operated mainly by private businessmen from China and India. It is then reportedly illegally exported to the world market via neighbouring Zambia.
Minister Ndongala said during recent years officials from North Korea and African countries expressed interest in the Congo uranium.
Out of reach
The government has declared the mine a no-go area.
However, the country's infrastructure has been crippled by seven years of war and the government's edicts have had little effect at the mine - which is 2,000km (1,250 miles) from the capital Kinshasa.
"We're very concerned. Congo is obliged to report any uranium mining activities as well as exports," IAEA spokeswoman Melissa Fleming told the BBC's Newshour programme.
"We are of course aware of the turbulent situation in terms of security and political situation in the region, we're aware that the government may not itself be fully aware of the activities ongoing in some parts of the country," she said.
"That said, we are demanding information from the government on this alleged illegal mining."
Diplomats based in Kinshasa have also expressed concern.
Uranium extracted from the Shinkolobwe mine was used by the Americans in the atomic bombs dropped on Hiroshima and Nagasaki in Japan during World War II.