The World Health Organization is monitoring a cluster of 10 cases of pneumonia from an unknown cause in Argentina in an outbreak that so far has included three deaths.

The cases are linked to a single private clinic in the city of San Miguel de Tucumán, located in the northwest part of the country, according to the Pan American Health Organization (Paho), the regional office of the WHO.

An initial report on Tuesday included five healthcare workers and a patient who was treated in the intensive care ward of the clinic, with symptoms emerging between Aug 18 and 22.

On Thursday, local health officials reported another three cases, bringing the total to nine, including three deaths. All three people who died had other health conditions.

On Friday, Argentina reported an additional case.

Symptoms have included fever, muscle and abdominal pain and shortness of breath. Several patients had pneumonia in both lungs.

Tests for known respiratory viruses and other viral, bacterial and fungal agents were all negative, Paho said. Biological samples have been sent to Argentina's National Administration of Laboratories and Health Institutes for additional testing, which will include an analysis for the presence of toxins.

Dr Michael Osterholm, an infectious disease expert at the University of Minnesota, said given that the lungs are heavily involved, the cause is likely something the patients inhaled.

He first suspected Legionnaires' disease, which is caused by inhaling droplets of water containing Legionella bacteria, but tests have ruled that out.

Paho and the WHO are monitoring the outbreak and assisting local health officials with the investigation.

Osterholm said "mystery illnesses" do sometimes happen, and most often they can be explained by some local outbreak that does not have pandemic implications.

Osterholm said he expects more definitive information from Argentine health officials in the next five to seven days.

- Reuters