The leak occurred shortly before noon at one of the Swiss pharmaceutical giant’s Schweizerhalle plants in Muttenz, cantonal police said.

Police said according to initial findings a few litres of a caustic liquid chemical spilled, releasing a gas that irritates the mucous membranes.

The six hospitalized people, five workers from an outside cleaning company and a Novartis employee, complained of a severe cough, police said.

Three of them were released from hospital within hours, while another seven people were medically treated on site.

The environment outside the production building was never threatened and the affected buildings were “vented” and returned to normal conditions, police said.

An investigation has been launched in to the cause of the spill.

In a statement police explained that in the canton of Basel Country, cantonal authorities exclusively provide information to the public on chemical spills.

According to the policy, Novartis cannot publicly comment on the incident.

The spill took place in the Schweizerhalle industrial area that straddles the adjoining municipalities of Muttenz and Pratteln.

The area is famous for being the site of one of the worst environmental disasters in recent European history.

In 1986, 1,351 tonnes of chemicals burned at a plant owned by Sandoz, which later created Novartis when it merged with Ciba-Geigy.

While people suffered no serious ill-effects from the fire, resulting toxic pollution that led to a devastating fish kill in the Rhine River took more than 10 years to clean up.