In the latest case, British officials alerted the World Health Organization on Saturday of the new virus in a man who transferred from Qatar to be treated in London. He had recently traveled to Saudi Arabia and is now being treated in an intensive care unit after suffering kidney failure.
Health officials don't know yet whether the virus could spread as rapidly as SARS did or if it might kill as many people.
"It's still (in the) very early days," said Gregory Hartl, a WHO spokesman. "At the moment, we have two sporadic cases and there are still a lot of holes to be filled in."
Hartl said it was unclear how the virus spreads. Coronaviruses are typically spread in the air but Hartl said scientists were considering the possibility that the patients were infected directly by animals. He said there was no evidence yet of any human-to-human transmission.
"All possible avenues of infection are being explored right now," he said.
So far there is no connection between the cases except for a history of travel in Saudi Arabia. SARS was first spread to humans from civet cats in China.
Hartl said no other countries have so far reported any similar cases to WHO.
Other experts said it was unclear how dangerous the virus is.