Hundreds of Belgian children underwent thyroid gland tests on their first day back at school after a radioactive leak in the vicinity of Charleroi city a week ago, the government said Monday.

Health authorities examined 350 school children so as to reassure families after radioactive iodine leaked from a medical laboratory in Fleurus town, said a spokesman for the government crisis centre administering the tests.

"The first results are negative. There is no problem," said the official, adding that more precise results would be made public Tuesday.

The incident -- which occurred August 23-24 at a laboratory that makes radioisotopes for medical imaging and cancer treatment -- put local residents on edge.

Officials initially said there was no risk -- but later urged precautionary measures.