Healthcare workers around the world should be alert for possible new cases of the novel coronavirus that has left one man critically ill and another dead, the World Health Organization said.

The agency issued an interim case definition for the novel virus, urging healthcare workers to be on the lookout for patients with unexplained respiratory illness combined with fever and cough, especially if they have recently visited Saudi Arabia or Qatar.

The WHO said no new cases have been confirmed, although a Danish hospital is reporting that it is treating five people with fever, coughing, and influenza-like symptoms.

Those admitted to the Odense University Hospital were a family of four where the father had been to Saudi Arabia, and an unrelated person who had been to Qatar, the news agency Agence-France Presse reported.

Officials of the hospital said they had obtained samples for testing and were hoping to get results soon.

The novel virus is a relative of the pathogen that caused the 2002-2003 outbreak of severe acute respiratory syndrome (SARS).

The WHO said in a statement the new pathogen "is a very different virus to SARS (but) given the severity of the two confirmed cases so far, WHO is engaged in further characterizing the novel coronavirus."

The critically ill patient, a 49-year-old man originally from Qatar, had recently travelled to Saudi Arabia and had become ill on Sept. 3. He was admitted to intensive care in Doha, Qatar, Sept. 7 with respiratory illness and renal failure before being airlifted to the U.K. on Sept. 11.

Genetic sequencing carried out by the U.K. Health Protection Agency found that the virus is related to the SARS pathogen, as well as others that cause milder disease, such as colds.

That sequencing also showed the virus is almost identical to one isolated from a Saudi man who died in that country earlier, the WHO said.

The discovery of the virus – and its association with Saudi Arabia – is raising concern about the annual Hajj pilgrimage to Mecca, which will begin next month, bringing millions of people to the Middle East.

The WHO said it is working closely with Saudi Arabia "to support the country's health measures for all visitors" taking part in the pilgrimage.

The agency's case definition includes criteria -- based on clinical, epidemiological and laboratory data -- for identifying a "patient under investigation," as well as "probable" and "confirmed" cases.

Under the guidelines: