LONDON (AP) -- Global health officials have alerted doctors to be on the lookout for a virus related to SARS (Severe Acute Respiratory Syndrome) but said there were no signs the disease was behaving like the syndrome that killed hundreds in 2003.

The World Health Organization announced this week that a new coronavirus had been found in a critically ill Qatari man who was being treated in London as well as in a Saudi Arabian man who died several months earlier. Genetic sequencing found the viruses in the two men to be nearly identical.

The two men might have been infected directly by animals, however, and there was no proof of human-to-human spread of the virus, WHO spokesman Gregory Hartl said Thursday.

"This is not SARS, it is a new virus and very distinct from SARS," he said. "There are still a lot of unanswered questions, so we cannot predict what might happen."

Hartl said WHO could not elaborate on the men's cases until investigations by national authorities were finished.

The new coronavirus is from a family of viruses that cause the common cold as well as SARS, the severe acute respiratory syndrome that killed about 800 people, mostly in Asia in a 2003 epidemic.

SARS jumped to people from civet cats and then mutated into a form easily spread among humans.

Unlike SARS, the new virus also causes rapid kidney failure, a complication not usually seen in respiratory viruses. In London, the Qatari patient is in critical but stable condition and is being treated with an artificial lung machine.

Hartl said WHO is monitoring reports of suspect cases but none have so far been confirmed. Britain's Health Protection Agency said a small number of potential cases were being evaluated. On Wednesday, Danish authorities ruled out a handful of cases.

WHO also issued a case definition for the new virus to help doctors spot cases early. The agency advised them to investigate any patients hospitalized with acute respiratory syndrome not explained by other causes who had close contact with probable cases or had recently traveled to Saudi Arabia or Qatar.

Saudi officials said they were concerned that the next month's annual Muslim Hajj pilgrimage -- which brings millions to Saudi Arabia from all around the world -- could allow the virus to spread. As a precautionary measure, they advised pilgrims to keep their hands clean and wear masks in crowded places.