In a statement, the Swiss defence department said that on Thursday afternoon, 43 recruits and members of the Jassbach academy in Linden, in Bern Canton, suddenly suffered from acute gastrointestinal problems, with diarrhoea and vomiting.

"As a result, all 43 service members were sent by ambulance and helicopter to hospitals and military medical facilities," the statement said.

An army spokesman told the ATS news agency that four of those affected were in a critical condition.

The authorities were still trying to determine the cause of the disease outbreak.

As a precaution, the canton of Bern has imposed a quarantine on the Jassbach site, which is specialised in cyber defence training, the statement said.