The team, including Congo's environment minister and experts from the country's atomic energy agency and environment ministry, was due in the city of Likasi later on Thursday.

Congolese authorities opened an inquiry on Wednesday after it was found that radioactive copper and cobalt ore might have been offloaded into the Mura river, 10 km (6 miles) from Likasi, a city with a population of 300,000.

Testing at the suspected dumping site on Thursday showed levels of radioactivity nearly 50 times the limit set for mineral exports from Congo, which is barred from exporting uranium, provincial Environment Minister Therese Lukenge said.

"Unfortunately, this is the same source that leads to a pumping station that distributes water to the population. This is the water that is delivered to the whole city," Lukenge told Reuters by telephone from the banks of the Mura river.

She said most of the minerals had already been swept downstream, towards a pumping station for the city's water supply which is operated by Congo's national copper and cobalt mining company Gecamines.

"We have begun taking samples at the site we pump from to see the level of contamination. But we have not stopped pumping ... We only learned about this last night," said Gecamines' operations director in Likasi, Dieudonne Nduwa.

ABANDONED MINE

The minerals were seized in Likasi last month and included 17 tonnes of copper ore with radiation levels 50 times the tolerable limit, which were destined for the Chinese firm Magma.

Orders had been given to transfer them to a nearby abandoned uranium mine last week as a safety precaution.

However, Katanga's provincial mines minister said on Wednesday the minerals never reached the mine and traces of the radioactive ore were discovered on a bridge spanning the Mura and along the river's banks.

"We will establish who is responsible, and once established we will arrest them. If the companies are in on it, we will take the appropriate measures," Congo's Environment Minister Didace Pembe said on Thursday.

Pembe said residents would be told not to use water from the river if test results showed dangerous levels of contamination.

Officials at Magma and at Congolese mining company Chemaf, from which smaller batches of less radioactive ore were taken, could not be reached for comment.

Ore mined in Katanga, home to one of the world's richest belts of copper and cobalt, habitually contains trace amounts of uranium and some foreign companies are believed to be particularly interested in these uranium-rich ores.