"There's a risk that the supply of these products will be constrained in the coming weeks," said Michel Boland, an executive at the Institut National des Radioelements.

IRE, which is based near the southern Belgian city of Charleroi, exports worldwide, Boland told AFP.

Production of radioisotopes was stopped on Monday after an abnormally high level of iodine was detected over the weekend in a ventilation chimney.

"The level of iodine in the environment is harmless to the health of employees and people living nearby because it was well below the legal limit," Boland said.

The Belgian federal nuclear watchdog gave the incident a three on an international scale of nuclear events that runs to seven, making it the most serious ever in Belgium.

However, the agency said that so far an action plan was not needed nor were particular measures to protect the environment.

"We suppose that a little bit of (radioactive) liquid was pumped by the ventilation system and that some entered the piping and was drawn into the chimney," Boland said, adding that inspections were under way.

He said that it was "impossible at this point" to say when production of radioisotopes would resume and what the impact of a halt would be.

However, the shortage could be made worse by the fact that French and Belgian reactors that supply the factory are in maintenance and that the Dutch reactor of Petten has also been stopped because of a problem, he said.