The UN health agency has raised concerns about a "large outbreak" of Legionnaire's disease in Portugal.

An outbreak of Legionnaire's disease in Portugal that has killed five people in less than a week constitutes a "major public health emergency", the World Health Organization has warned.

The UN health agency said on Tuesday that Portugal had notified it of a "large outbreak" of Legionnaire's disease in the Vila Franca de Xira suburb of Lisbon, voicing concern about the public health implications.

The disease, a form of pneumonia, is not contagious and cannot be spread directly from person to person, but can multiply in water and air conditioning systems, including humidifiers, whirlpools and spas.

It is usually caught by breathing in small droplets of contaminated water.

The Portuguese government said the outbreak was very likely caused by bacteria found in the refrigeration system of a fertiliser factory in Vila Franca de Xira, about 30km from Lisbon.

Environment Minister Jorge Moreira da Silva said the Adubos factory was also being tested to see if there was negligence in maintenance and if any "environmental crime" had been committed.

He said the outbreak was the third biggest in the history of the disease.

The first 17 Portuguese cases were identified late last Thursday, and since then a full 233 cases have been reported, WHO spokesman Christian Lindmeier told reporters in Geneva, adding that most of those infected were aged between 50 and 60.

Thirty-eight people are in intensive care and five have died, most of them with pre-existing medical conditions.

The disease was discovered in the United States in 1976 during a convention of the American Legion, a military veterans group, at which 29 people died.

Copyright AAP 2014.