The World Health Organisation issued a global alert on Monday for a new SARS-like respiratory virus which left a man from Qatar critically ill in a London hospital and killed at least one more in Saudi Arabia.
The 49-year-old Qatari was admitted to an intensive care unit in Doha on September 7 suffering from acute respiratory infection and kidney failure before being transferred to Britain by air ambulance on September 11, the WHO said.
A Saudi Arabian national died earlier this year from a virtually identical virus, the WHO said, while Saudi medical authorities said they were investigating other possible cases of the disease.
The WHO confirmed the illness was in the coronavirus family but was not SARS, or Severe Acute Respiratory Syndrome, which swept out of China in 2003, killing more than 800 people worldwide.
"This is a new virus," WHO spokesman Gregory Hartl told AFP.
"We haven't heard of any more new cases. We don't have an appreciation of how widespread the virus is," Hartl said. "This is one reason why we're trying to get more information. We don't know how it's transmitted."
The WHO said the Qatari first fell ill on September 3 after visiting Saudi Arabia.
Britain's Health Protection Agency confirmed the presence of the new coronavirus and then found that it was a 99.5 percent match with a virus obtained from the lung tissue of a 60-year-old Saudi man who died earlier this year.
Coronaviruses are causes of the common cold but can also include more severe illnesses including SARS.
In Riyadh, the health ministry revealed that a total of three people, including the Qatari man, had been diagnosed with the virus after spending time in Saudi Arabia, according to state media. The other two later died.
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The ministry said it would continue to "follow developments" linked to the disease "in coordination with international health organisations," adding that "these are rare cases and the situation is reassuring".
The announcement comes ahead of next month's annual Muslim hajj pilgrimage to Saudi Arabia's holy city of Mecca which will attract nearly three million believers, although the WHO said it did not recommend any travel restrictions.
In Britain, the HPA, an organisation set up by the government to manage infectious diseases, meanwhile, stressed no-one else in Britain, including those who had come into contact with the man, were reporting symptoms.
The HPA said the new virus was "different from any that have previously been identified in humans."
It said there were encouraging signs that it was not as infectious as SARS as there had been no evidence of illness in people who had been in contact with the Qatari or the Saudi, including in health workers.
"Based on what we know about other coronaviruses, many of these contacts will already have passed the period when they could have caught the virus from the infected person," it said.
John Watson, head of the respiratory diseases at the HPA, said: "Immediate steps have been taken to ensure that people who have been in contact with the UK case have not been infected, and there is no evidence to suggest they have."
Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, urged caution, saying any evidence of human-to-human transmission causing severe disease "would be very worrying".
But fellow expert John Oxford, professor at the University of London, said he was "somewhat relaxed" because he believed the illness was more likely to behave "like a nasty infection rather than join the 'exception' group like SARS."
"This new virus does not to me appear to be in the same 'big bang' group," he added. "I am very pleased that it does not!"
SARS (Severe Acute Respiratory Disease), which mainly affected Asia, was recognised at the end of February 2003.